AP_gtest: enable testing for exit conditions, SITL unit tests

tests/AP_gtest.h

@@ -5,6 +5,7 @@
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 
 #include <gtest/gtest.h>
+#include <gtest/gtest-spi.h>
 
 
 #define AP_GTEST_PRINTATBLE_PARAM_MEMBER(class_name_, printable_member_) \
@@ -14,6 +15,27 @@ ::std::ostream& operator<<(::std::ostream& os, const class_name_& param) \
     return os << param.printable_member_; \
 }
 
+/*
+* Override the WEAK version of AP_HAL_SITL/system.cpp panic() instead of staying in an infinite loop
+* This is used by the gtest suite to test for an exit signal caused by a test statement and continue testing
+* Printing to stderr  is required for gtest matching
+*/
+#define AP_GTEST_PANIC() \
+void AP_HAL::panic(const char *errormsg, ...) \
+{ \
+    va_list ap; \
+    auto outputs = {stdout, stderr}; \
+    for( auto output : outputs) { \
+        fflush(stdout); \
+        fprintf(output, "PANIC: "); \
+        va_start(ap, errormsg); \
+        vfprintf(output, errormsg, ap); \
+        va_end(ap); \
+        fprintf(output, "\n"); \
+    } \
+    abort(); \
+}
+
 #define AP_GTEST_MAIN() \
 int main(int argc, char *argv[]) \
 { \