No need to print stack trace to the file when running tests (that actually...

No need to print stack trace to the file when running tests (that actually generate errors on purpose).

mx.fastr/mx_fastr.py

@@ -389,6 +389,8 @@ def _junit_r_harness(args, vmArgs, junitArgs):
 
     # on some systems a large Java stack seems necessary
     vmArgs += ['-Xss12m']
+    # no point in printing errors to file when running tests (that contain errors on purpose)
+    vmArgs += ['-DR:-PrintErrorStacktracesToFile']
 
     vmArgs += _graal_options(nocompile=True)