Generalize (testdata) substitution in end2end tests

We replace the `(testdata)` placeholder with `testutils.abs_datapath()` in a few end2end
tests. So far we only needed to replace `(testdata)` with an OS path (e.g. in
`tests/end2end/features/spawn.feature`). By introducing `(testdata)` to an end2end test
in `tests/end2end/features/hints.feature`, we required a new use case: replacing
`(testdata)` as part of a valid file:// URI.

```
When I open file://(testdata)/some/file.txt
```

Replacing `(testdata)` in above BDD step with a plain OS path resulted in invalid URIs,
e.g. for the path "C:\\Users" above BDD step results in this invalid URI:

```
When I open file://C:\Users/some/file.txt
```

We deal with this by first isolating the `(testdata)` substitution in a single place.
Having `(testdata)` substitutions in a single place, we simply special-case the substitution
of file:// paths, which will be part of a URI.
Successful substitution for above BDD step looks like the following:

```
When I open file:///C:/Users/some/file.txt
```

tests/end2end/features/conftest.py

@@ -198,7 +198,7 @@ def open_path(quteproc, server, path):
     - With "... as a URL", it's opened according to new_instance_open_target.
     """
     path = path.replace('(port)', str(server.port))
-    path = path.replace('(testdata)', os.fspath(testutils.abs_datapath()))
+    path = testutils.substitute_testdata(path)
 
     new_tab = False
     new_bg_tab = False
@@ -270,7 +270,7 @@ def run_command(quteproc, server, tmpdir, command):
         invalid = False
 
     command = command.replace('(port)', str(server.port))
-    command = command.replace('(testdata)', str(testutils.abs_datapath()))
+    command = testutils.substitute_testdata(command)
     command = command.replace('(tmpdir)', str(tmpdir))
     command = command.replace('(dirsep)', os.sep)
     command = command.replace('(echo-exe)', _get_echo_exe_path())
@@ -364,7 +364,7 @@ def hint(quteproc, args):
 
 @bdd.when(bdd.parsers.parse('I hint with args "{args}" and follow {letter}'))
 def hint_and_follow(quteproc, args, letter):
-    args = args.replace('(testdata)', str(testutils.abs_datapath()))
+    args = testutils.substitute_testdata(args)
     args = args.replace('(python-executable)', sys.executable)
     quteproc.send_cmd(':hint {}'.format(args))
     quteproc.wait_for(message='hints: *')

tests/helpers/testutils.py

@@ -196,6 +196,24 @@ def abs_datapath():
     return path.resolve()
 
 
+def substitute_testdata(path):
+    r"""Replace the (testdata) placeholder in path with `abs_datapath()`.
+
+    If path is starting with file://, return path as an URI with file:// removed. This
+    is useful if path is going to be inserted into an URI:
+
+    >>> path = substitute_testdata("C:\Users\qute")
+    >>> f"file://{path}/slug  # results in valid URI
+    'file:///C:/Users/qute/slug'
+    """
+    if path.startswith('file://'):
+        testdata_path = abs_datapath().as_uri().replace('file://', '')
+    else:
+        testdata_path = str(abs_datapath())
+
+    return path.replace('(testdata)', testdata_path)
+
+
 @contextlib.contextmanager
 def nop_contextmanager():
     yield