Refactor individual tmpdir fixtures.

I was adding a downloads one and saw the opportunity to refactor. Didn't
end up saving that much because I couldn't figure out how to get the
method that does the work to use the fixtures while also taking an
argument, so they have to be passed through from the calling ones. Oh
well.

tests/helpers/fixtures.py

@@ -459,16 +459,35 @@ def mode_manager(win_registry, config_stub, qapp):
     objreg.delete('mode-manager', scope='window', window=0)
 
 
+def standarddir_tmpdir(folder, monkeypatch, tmpdir):
+    """Set tmpdir/config as the configdir.
+
+    Use this to avoid creating a 'real' config dir (~/.config/qute_test).
+    """
+    confdir = tmpdir / folder
+    confdir.ensure(dir=True)
+    if hasattr(standarddir, folder):
+        monkeypatch.setattr(standarddir, folder,
+                            lambda **_kwargs: str(confdir))
+    return confdir
+
+
+@pytest.fixture
+def download_tmpdir(monkeypatch, tmpdir):
+    """Set tmpdir/download as the downloaddir.
+
+    Use this to avoid creating a 'real' download dir (~/.config/qute_test).
+    """
+    return standarddir_tmpdir('download', monkeypatch, tmpdir)
+
+
 @pytest.fixture
 def config_tmpdir(monkeypatch, tmpdir):
     """Set tmpdir/config as the configdir.
 
     Use this to avoid creating a 'real' config dir (~/.config/qute_test).
     """
-    confdir = tmpdir / 'config'
-    confdir.ensure(dir=True)
-    monkeypatch.setattr(standarddir, 'config', lambda auto=False: str(confdir))
-    return confdir
+    return standarddir_tmpdir('config', monkeypatch, tmpdir)
 
 
 @pytest.fixture
@@ -477,10 +496,7 @@ def data_tmpdir(monkeypatch, tmpdir):
 
     Use this to avoid creating a 'real' data dir (~/.local/share/qute_test).
     """
-    datadir = tmpdir / 'data'
-    datadir.ensure(dir=True)
-    monkeypatch.setattr(standarddir, 'data', lambda system=False: str(datadir))
-    return datadir
+    return standarddir_tmpdir('data', monkeypatch, tmpdir)
 
 
 @pytest.fixture
@@ -489,10 +505,7 @@ def runtime_tmpdir(monkeypatch, tmpdir):
 
     Use this to avoid creating a 'real' runtime dir.
     """
-    runtimedir = tmpdir / 'runtime'
-    runtimedir.ensure(dir=True)
-    monkeypatch.setattr(standarddir, 'runtime', lambda: str(runtimedir))
-    return runtimedir
+    return standarddir_tmpdir('runtime', monkeypatch, tmpdir)
 
 
 @pytest.fixture
@@ -501,10 +514,7 @@ def cache_tmpdir(monkeypatch, tmpdir):
 
     Use this to avoid creating a 'real' cache dir (~/.cache/qute_test).
     """
-    cachedir = tmpdir / 'cache'
-    cachedir.ensure(dir=True)
-    monkeypatch.setattr(standarddir, 'cache', lambda: str(cachedir))
-    return cachedir
+    return standarddir_tmpdir('cache', monkeypatch, tmpdir)
 
 
 @pytest.fixture