Add support for #@elif clauses in Test files

[reiniscirpons]

This PR adds the behaviour requested in #5652 by supporting #@elif clauses in test files.

Resolves #5652

Text for release notes

See title.

Further details

The behaviour is implemented by modifying the existing state machine used for parsing #@if ... #@else ... #@fi statements in ParseTestInput. This is done in a way where the state transitions are identical to what they were prior to this PR in tests using only #@if and #@else clauses. So, this change should be backwards compatible with existing test cases and shouldn't introduce new bugs in test cases that do not use #@elif.

The only other potential issue might be in test cases that have #@elif as an output line, e.g.

gap> Print("#@elif 1+1 = 2\n");
#@elif 1+1 = 2

is a valid test case prior to this PR, but after this PR it would lead to a test case syntax error when parsing. I was not able to find any such instances in the gap codebase, or the codebase of any packages we distribute. So I think its ok to call the change backwards compatible.

[ChrisJefferson]

Thanks!

[ChrisJefferson]

I slightly tweaked the title, just so it could be used directly in the release notes (which are plain text)

[fingolfin]

@ChrisJefferson actually the release notes are not plaintext, they use MarkDown, so it was better before...

[fingolfin]

Great!