cl20: Reuse test harness code in printf

Some of the setup functionality is already there in the test harness, so
use that and remove the duplicated code from within the suite.

Signed-off-by: Radek Szymanski <radek.szymanski@arm.com>

test_conformance/printf/test_printf.h

@@ -119,7 +119,7 @@ size_t verifyOutputBuffer(char *analysisBuffer,testCase* pTestCase,size_t testId
 #define checkNull(ptr, msg)            \
     if (!ptr) {                    \
     log_error("%s failed\n", msg);        \
-    return -1;                \
+    return TEST_FAIL;                \
     }
 
 // When a helper returns a negative one, we want to return from main