test: libtest: Add test return codes

The meson test infrastructure uses return codes to determine test
results. Define these values for use in tests.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[Use an enum instead of macros for test return codes.]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

test/libtest/test.h

@@ -9,6 +9,12 @@
 
 #include <sstream>
 
+enum TestStatus {
+	TestPass = 0,
+	TestFail = -1,
+	TestSkip = 77,
+};
+
 class Test
 {
 public: