tests: Enable unit test on Windows

Allow run some unit tests on Windows.
* Remove hardcoded path separator in whitelist env for Meson to choose
  OS-specific separator automatically (i.e., ';' for windows and ':' for *nix)
* Add dependency explicitly for some test cases, otherwise plugins couldn't be
  loaded on uninstalled environment of Windows.
6 jobs for windows-unittest in 33 minutes and 5 seconds (queued for 14 minutes and 58 seconds)
Status Job ID Name Coverage
  Preparation
passed #66781
gst indent

00:00:33

passed #66780
manifest

00:00:22

 
  Build
passed #66782
build fedora x86_64

00:03:49

 
  Test
passed #66783
check fedora

00:16:51

 
  Full Builds
passed #66784
build android arm64 api28

00:02:21

passed #66785
build cerbero fedora x86_64

00:11:28