tests: Re-enable isoff test on Windows

... and drop libxml2 dependency since it never be used by
both test and gstisoff.
6 jobs for windows-unittest in 30 minutes and 18 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed gst indent #66939

00:00:32

passed manifest #66938

00:00:22

 
  Build
passed build fedora x86_64 #66940

00:03:06

 
  Test
passed check fedora #66941

00:15:43

 
  Full Builds
passed build android arm64 api28 #66942

00:03:14

passed build cerbero fedora x86_64 #66943

00:10:43