[gdb/testsuite] Minimize gdb restarts

Minimize gdb restarts, applying the following rules:
- don't use prepare_for_testing unless necessary
- don't use clean_restart unless necessary

Also, if possible, replace build_for_executable + clean_restart
with prepare_for_testing for brevity.

Touches 68 test-cases.

Tested on x86_64-linux.
68 files changed