Extend the interrupt-flush mechanism already used for cmakeBuild and
cmakeInstall to ctest, so that a user interrupt (e.g. Ctrl+C) still writes
the overall ctest instrumentation snippet, marked with interruptSignal,
before exiting.
On interrupt CTest stops scheduling further tests at every launch site,
skips the postCTest hook, forces a non-zero exit status, and preserves the
checkpoint file so that a later 'ctest -F' can resume the interrupted test
set.
Issue: #27859