Optionally install examples and tests

This makes it easier to test PipeWire in its "as-installed" state,
for example in an OS distribution.

The .test metadata files in ${datadir}/installed-tests/${package} are
a convention taken from GNOME's installed-tests initiative, allowing a
generic test-runner like gnome-desktop-testing to discover and run tests
in an automatic way.

The installation path ${libexecdir}/installed-tests/${package} is also
a convention borrowed from GNOME's installed-tests initiative.

In addition to the automated tests, I've installed example executables
in the same place, for manual testing. They could be separated into
a different directory if desired, but they seem like they have more
similarities with the automated tests than differences: both are there
to test that PipeWire works correctly, and neither should be relied on
for production use. Some examples are installed in deeper subdirectories
to avoid name clashes.

Signed-off-by: Simon McVittie <smcv@debian.org>

meson_options.txt

@@ -14,6 +14,10 @@ option('tests',
        description: 'Build tests',
        type: 'boolean',
        value: true)
+option('installed_tests',
+       description: 'Install manual and automated test executables',
+       type: 'boolean',
+       value: false)
 option('gstreamer',
        description: 'Build GStreamer plugins',
        type: 'boolean',