TestsManager: _add_blacklist in set_default_blacklist

Otherwise test suites that want to set_default_blacklist and
add tests in setup_tests were seeing their blacklist ignored.

Split up and rename set_blacklists() to complete the refactoring
11 jobs for misc-improvements in 52 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #242726
gst indent

00:00:17

passed #242725
manifest

00:00:38

 
  Build
passed #242727
build fedora x86_64

00:04:33

passed #242728
build nodebug fedora x86_64

00:04:05

 
  Test
passed #242729
check fedora

00:02:59

passed #242730
integration testsuites fedora

00:25:44

 
  Full Builds
passed #242732
build cerbero android universal

00:14:31

passed #242733
build cerbero cross win32

00:09:45

passed #242734
build cerbero cross win64

00:07:29

passed #242731
build cerbero fedora x86_64

00:08:23

 
  Apps
passed #242735
android universal examples

00:06:59