Let the Windows job read the merged suite

The merge put a Windows runner under tests master wrote for two systems,
and 21 of them fell over on it. Two were the quoting: `command_for` goes
through shlex now, so a Windows path comes back in quotes, and the two
tests that read the command as a string were reading it as a Linux one.
They ask through the same join instead.

The other nineteen are `integrate.py`, which writes the menu entry and
the login item a downloaded build installs for itself. There are two
downloads, an AppImage and a disk image, so the module has a Linux half
and a macOS half and nothing a Windows host would run: its tests hand
the fake home over in $HOME, which Windows does not read, and compare
paths that start at the root. They carry `@posix_only`, which comes off
again the day there is a Windows build to integrate.

CONTRIBUTING.md

@@ -70,7 +70,7 @@ there can tell that apart from an empty device list, which only means the tool
 that lists them is not installed.
 
 The tests are split along the same line, and almost none of them are skipped.
-1104 of the 1147 run on any machine, including every line of the Wayland, X11,
+1084 of the 1147 run on any machine, including every line of the Wayland, X11,
 macOS and Windows backends: the programs are faked at `shutil.which`, the
 frameworks and system libraries at the one function that loads them
 (`paste._win_api`, `hotkey._win_input`). A test class says which system it is
@@ -88,8 +88,11 @@ is sitting at. What the systems owe in common is written once as a contract
 class and subclassed by each of them.
 
 The 43 that do carry `@linux_only` are the ones that would need the real thing:
-the `/dev/input` listener, KDE's shortcut file, GNOME's gsettings. Mark a test
-that way only when faking it would leave nothing to test. A test that quietly
+the `/dev/input` listener, KDE's shortcut file, GNOME's gsettings. The 20 with
+`@posix_only` are `integrate.py`, the menu entry and the login item a downloaded
+build writes for itself: there are two downloads, an AppImage and a disk image,
+so that module has no Windows half for a Windows host to check. Mark a test
+either way only when faking it would leave nothing to test. A test that quietly
 stops running on the platform you are porting to protects nothing.
 
 ## What a pull request should carry