cmd/gogio: groundwork for Windows e2e tests on Wine

First, move from debian unstable to testing, since sway was promoted to
testing as of earlier this week.

Second, use the --sync option when using xdotool to move an X11 mouse.
This makes the command block until the mouse has finished moving to the
specified location, removing a potential race with the following
'xdotool click' command.

Third, deduplicate some logic into driverBase: tempDir to create a
temporary directory within a test, and needPrograms to skip a test if
the required programs aren't available.

Lastly, split the code that starts the X11 server into a method, so that
the future Wine e2e driver can reuse it. Since Wine is tightly coupled
with X11, we can reuse a good part of the code, including the X11 server
and the xdotool mechanisms.

We also add a TODO to perhaps improve the handling of the app's output
under each of the e2e test cases.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

.builds/linux.yml

@@ -1,4 +1,4 @@
-image: debian/unstable # TODO(mvdan): switch back to testing once sway hits that repo
+image: debian/testing
 packages:
  - curl
  - pkg-config