Adds tests for buffer command.

The buffer_troubling_args tests may look a little un-intuitive but that is
because they are testing the edge cases for the current behaviour. If these
edge cases are encountered during normal usage you are doing something wrong.

tests/integration/features/tabs.feature

@@ -709,3 +709,116 @@ Feature: Tab management
             - data/hints/link.html
             - about:blank
             - data/hello.txt (active)
+
+    # :buffer
+
+    Scenario: buffer without args
+        Given I have a fresh instance
+        When I run :buffer
+        Then the error "buffer: The following arguments are required: index" should be shown
+
+    Scenario: buffer one window title present
+        When I open data/title.html
+        And I open data/search.html in a new tab
+        And I open data/scroll.html in a new tab
+        And I run :buffer "Searching text"
+        Then the following tabs should be open:
+            - data/title.html
+            - data/search.html (active)
+            - data/scroll.html
+
+    Scenario: buffer one window title not present
+        When I run :buffer "invalid title"
+        Then the error "No matching tab for: invalid title" should be shown
+
+    Scenario: buffer two window title present
+        When I open data/title.html
+        And I open data/search.html in a new tab
+        And I open data/scroll.html in a new tab
+        And I open data/caret.html in a new window
+        And I open data/paste_primary.html in a new tab
+        And I run :buffer "Scrolling"
+        Then the session should look like:
+            windows:
+            - active: true
+              tabs:
+              - history:
+                - url: about:blank
+                - url: http://localhost:*/data/title.html
+              - history:
+                - url: http://localhost:*/data/search.html
+              - active: true
+                history:
+                - url: http://localhost:*/data/scroll.html
+            - tabs:
+              - history:
+                - url: http://localhost:*/data/caret.html
+              - active: true
+                history:
+                - url: http://localhost:*/data/paste_primary.html
+
+    Scenario: buffer one window index not present
+        When I open data/title.html
+        And I run :buffer "666"
+        Then the error "There's no tab with index 666!" should be shown
+
+    Scenario: buffer one window win not present
+        When I open data/title.html
+        And I run :buffer "2/1"
+        Then the error "There's no window with id 2!" should be shown
+
+    Scenario: buffer two window index present
+        Given I have a fresh instance
+        When I open data/title.html
+        And I open data/search.html in a new tab
+        And I open data/scroll.html in a new tab
+        And I run :open -w http://localhost:(port)/data/caret.html
+        And I open data/paste_primary.html in a new tab
+        And I wait until data/caret.html is loaded
+        And I run :buffer "0/2"
+        Then the session should look like:
+            windows:
+            - active: true
+              tabs:
+              - history:
+                - url: about:blank
+                - url: http://localhost:*/data/title.html
+              - active: true
+                history:
+                - url: http://localhost:*/data/search.html
+              - history:
+                - url: http://localhost:*/data/scroll.html
+            - tabs:
+              - history:
+                - url: http://localhost:*/data/caret.html
+              - active: true
+                history:
+                - url: http://localhost:*/data/paste_primary.html
+
+    Scenario: buffer troubling args 01
+        Given I have a fresh instance
+        When I open data/title.html
+        And I run :buffer "-1"
+        Then the error "There's no tab with index -1!" should be shown
+
+    Scenario: buffer troubling args 02
+        When I open data/title.html
+        And I run :buffer "/"
+        Then the following tabs should be open:
+            - data/title.html (active)
+
+    Scenario: buffer troubling args 03
+        When I open data/title.html
+        And I run :buffer "//"
+        Then the following tabs should be open:
+            - data/title.html (active)
+
+    Scenario: buffer troubling args 04
+        When I open data/title.html
+        And I run :buffer "0/x"
+        Then the error "No matching tab for: 0/x" should be shown
+
+    Scenario: buffer troubling args 05
+        When I open data/title.html
+        And I run :buffer "1/2/3"
+        Then the error "No matching tab for: 1/2/3" should be shown