ci: run tests under gdb on Linux

[madebr]

• By running them under the gdb debugger, a stacktrace can be printed when a fatal signal is emitted.
• The pr also contains a commit adding the possibility to override ctest arguments. This is useful to stress test a particular test.
  Example: adding [sdl-ci-ctest-arg --repeat until-fail:400000 -VV -R torturethread -j2] to your commit message will repeat torturethead maximum 400000 times until it fails. This argument should be combined with [sdl-ci-filter X] to limit the number of jobs that are started.

Description

Existing Issue(s)

[madebr]

For a reason unknown to me, the Steam Linux Runtime 3.0 (x86_64 and arm64) jobs fail to find gdb.

[madebr]

Also, can somebody test whether -DSDLTEST_GDB=ON also works on Apple? Add an abort() somewhere in SDL's code and check whether ctest -V -R <test-name> prints a backtrace.