test-run: avoid testing cases of filesystem binding that won't work
test-run: avoid testing cases of filesystem binding that won't work

This test effectively assumed that ${test_builddir} is in /home, /opt,
a non-FHS top level directory, or some other directory that isn't
in dont_mount_in_root[] in flatpak-run.c. In a distro build where
${libexecdir} is below /usr, when running the installed-tests
so ${test_builddir} is ${libexecdir}, this assumption doesn't hold.

We can't just copy the file we're dealing with into a subdirectory
of /tmp or /var/tmp either, because those directories aren't
part of --filesystem=host. Meanwhile, we also don't want to pollute
$HOME with test detritus.

For the best possible coverage given all those constraints, we try
three directories: ${test_builddir}; ${TEST_DATA_DIR}, which is
in /var/tmp; and ~/.flatpak-tests, but only if it already exists.

When testing --[no]filesystem=host overrides, we don't even try
${TEST_DATA_DIR}, because we know it's in /var/tmp which can only
be shared explicitly, but we do try the others.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@debian.org>

handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,amigadave/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,amigadave/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,amigadave/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak