Class File now validates relative paths

The code from function validate in class UserStyleSheet has been migrated to
class File. One test had to be modified due to different expected behaviour.

qutebrowser/config/configtypes.py

@@ -818,6 +818,10 @@ class File(BaseType):
                 raise configexc.ValidationError(value, "may not be empty!")
         value = os.path.expanduser(value)
         try:
+            if not os.path.isabs(value):
+                abspath = os.path.join(standarddir.config(), value)
+                if os.path.isfile(abspath):
+                    return
             if not os.path.isfile(value):
                 raise configexc.ValidationError(value, "must be a valid file!")
             if not os.path.isabs(value):
@@ -1177,11 +1181,11 @@ class UserStyleSheet(File):
                 raise configexc.ValidationError(value, "may not be empty!")
         value = os.path.expandvars(value)
         value = os.path.expanduser(value)
+        try:
+            super().validate(value)
+        except configexc.ValidationError:
             try:
                 if not os.path.isabs(value):
-                abspath = os.path.join(standarddir.config(), value)
-                if os.path.isfile(abspath):
-                    return
                     # probably a CSS, so we don't handle it as filename.
                     # FIXME We just try if it is encodable, maybe we should
                     # validate CSS?
@@ -1191,8 +1195,6 @@ class UserStyleSheet(File):
                     except UnicodeEncodeError as e:
                         raise configexc.ValidationError(value, str(e))
                     return
-            elif not os.path.isfile(value):
-                raise configexc.ValidationError(value, "must be a valid file!")
             except UnicodeEncodeError as e:
                 raise configexc.ValidationError(value, e)
 

tests/config/test_configtypes.py

@@ -1378,7 +1378,6 @@ class TestFile:
         os_path.expanduser.side_effect = lambda x: x
         os_path.isfile.return_value = True
         os_path.isabs.return_value = False
-        with pytest.raises(configexc.ValidationError):
         self.t.validate('foobar')
 
     def test_validate_expanduser(self, os_path):