CI: Run tests/v4 separately from whole test suite and disable MacOS and Windows tests

[VeckoTheGecko]

• Chose the correct base branch (v4-dev for v4 changes)
• Fixes None

Changes:

• Add test to matrix so that tests/v4 is also run separately by itself
  • This check makes it easier to parse whether a set of changes has broken other functionality that has been added in v4 (which is more pressing than test failures from old v3 tests)
• Disable tests on Windows and Mac
  • These will be introduced again at a later stage when they're of interest again. This is mainly to reduce CI wait times and resource usage.

[VeckoTheGecko]

@fluidnumerics-joe just updated this so it can be merged before #1980 . CI failures is due to some things in #1980. Should we go ahead and merge so you can rebase?

EDIT: Will go ahead and merge :)