Fix signalling test child threads crashing test 0

A thread made inside a test does not share the test index of its parent,
so any time one of those threads failed an assert, it would tell the
runner to shutdown test index zero.

core/testing/signal_handler_libc.odin

@@ -26,6 +26,8 @@ import "core:os"
 
 @(private="file", thread_local)
 local_test_index: libc.sig_atomic_t
+@(private="file", thread_local)
+local_test_index_set: bool
 
 // Windows does not appear to have a SIGTRAP, so this is defined here, instead
 // of in the libc package, just so there's no confusion about it being
@@ -45,6 +47,13 @@ stop_runner_callback :: proc "c" (sig: libc.int) {
 
 @(private="file")
 stop_test_callback :: proc "c" (sig: libc.int) {
+	if !local_test_index_set {
+		// We're a thread created by a test thread.
+		//
+		// There's nothing we can do to inform the test runner about who
+		// signalled, so hopefully the test will handle their own sub-threads.
+		return
+	}
 	if local_test_index == -1 {
 		// We're the test runner, and we ourselves have caught a signal from
 		// which there is no recovery.
@@ -114,6 +123,7 @@ This is a dire bug and should be reported to the Odin developers.
 
 _setup_signal_handler :: proc() {
 	local_test_index = -1
+	local_test_index_set = true
 
 	// Catch user interrupt / CTRL-C.
 	libc.signal(libc.SIGINT, stop_runner_callback)
@@ -135,6 +145,7 @@ _setup_signal_handler :: proc() {
 
 _setup_task_signal_handler :: proc(test_index: int) {
 	local_test_index = cast(libc.sig_atomic_t)test_index
+	local_test_index_set = true
 }
 
 _should_stop_runner :: proc() -> bool {