tests: Use a temporary directory for testfilemonitor

Previously, its tests were being run in the build directory, which is
fine (it should always be writable). If multiple tests were run in
parallel, for example with Meson’s `--repeat` option, their test files
would collide.

Fix that by running each test instance in a separate subdirectory of
`/tmp`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1634
9 jobs for 1634-testfilemonitor-race in 15 minutes and 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #322792
cross-android_api21_arm64

00:01:54

passed #322793
cross-android_api28_arm64

00:01:47

passed #322794
cross-mingw64

00:02:31

passed #322790
debian-stable-x86_64

00:02:33

passed #322789
fedora-x86_64

00:06:08

passed #322791
G_DISABLE_ASSERT

00:02:32

passed #322795
win32
msys2-mingw32

00:13:22

passed #322796
win32
vs2017-x64

00:09:44

 
  Coverage
passed #322797
coverage

00:02:11

78.1%