testament: remove deadcode related to realtimeGC, testC, callCCompiler

[timotheecour]

I keep running into testament bugs, so simplifying the code is worthwhile (will also simplify other PRs including upcoming custom megatests PR timotheecour#726)

while writing #18086, i noticed that callCCompiler is actually deadcode; this hasn't been run in a while, and there is no need to revive it as there would be better/simpler ways to do it nowadays if ever needed (e.g. running c code from a nim file, etc).

the only place where this is called is:
testC r, makeTest("tests/realtimeGC/cmain", cOptions, cat), actionRun
which isn't run unless you call:
XDG_CONFIG_HOME= nim r --lib:lib --stacktrace:on testament/testament.nim r longgc

and this fails since at least nim 1.0 with:

  testament/testament.nim(851) testament
  testament/testament.nim(822) main
  testament/categories.nim(713) processCategory
  testament/categories.nim(189) longGCTests
  testament/testament.nim(644) makeTest
  testament/specs.nim(251) parseSpec
  testament/specs.nim(184) extractSpec
  lib/system/io.nim(861) readFile
  Error: unhandled exception: cannot open: tests/realtimeGC/cmain.nim [IOError]

Also there were other issues with that code eg:

• c[5 .. ^1] is too magical (and, well, doesn't work)
• not tested in CI
• longgc is redundant with realtimeGC (which I've revived in fix realtimeGC tests; testament: error instead of echo when testament category is empty;  #16162 so it runs in CI)
• it referred to tests/realtimeGC/nmain which doesn't exist
• it uses "--threads: on" instead of "--threads:on"
• cmain.c wasn't portable, plus lots of other issues

but the main reason for removal is that it doesn't need to be revived, there are better ways if such need arises.

note

see also changes to specialCategories

[Araq]

Ready to go once the conflict has been resolved.