KVM: selftests: Report stacktraces SIGBUS, SIGSEGV, SIGILL, and SIGFPE by default

KVM: selftests: Report stacktraces SIGBUS, SIGSEGV, SIGILL, and SIGFPE by default

Register handlers for signals for all selftests that are likely happen due
to test (or kernel) bugs, and explicitly fail tests on unexpected signals
so that users get a stack trace, i.e. don't have to go spelunking to do
basic triage.

Register the handlers as early as possible, to catch as many unexpected
signals as possible, and also so that the common code doesn't clobber a
handler that's installed by test (or arch) code.

Tested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Tested-by: Shivank Garg <shivankg@amd.com>
Link: https://lore.kernel.org/r/20251016172853.52451-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>