test: support non-host tests

For example, for running tests with -target=wasm or
-target=cortex-m-qemu. It looks at the output to determine whether tests
were successful in the absence of a status code.

src/runtime/runtime_cortexm_qemu.go

@@ -21,6 +21,8 @@ func postinit() {}
 func main() {
 	preinit()
 	run()
+
+	// Signal successful exit.
 	arm.SemihostingCall(arm.SemihostingReportException, arm.SemihostingApplicationExit)
 	abort()
 }
@@ -54,3 +56,13 @@ func putchar(c byte) {
 func waitForEvents() {
 	arm.Asm("wfe")
 }
+
+func abort() {
+	// Signal an abnormal exit.
+	arm.SemihostingCall(arm.SemihostingReportException, arm.SemihostingRunTimeErrorUnknown)
+
+	// Lock up forever (should be unreachable).
+	for {
+		arm.Asm("wfi")
+	}
+}