selftest/trial_test: Fix repr(RuntimeError) changing in new python version

If using python 3.8.2, the trailing comma at the end of parameters is
not longer there (probably got fixed, since it's not needed). That
change breaks expected output.

Change-Id: I6da3024f946c0e761099058e812c0eacf3d6071f
diff --git a/selftest/trial_test.py b/selftest/trial_test.py
index 41d71e6..a99428a 100755
--- a/selftest/trial_test.py
+++ b/selftest/trial_test.py
@@ -37,13 +37,13 @@
 try:
     t.verify()
 except RuntimeError as e:
-    print('ok, got %r' % e)
+    print('ok, got RuntimeError: %s' % str(e))
 
 print('- detect missing file')
 t = Trial(d.child('missing_file'))
 try:
     t.verify()
 except RuntimeError as e:
-    print('ok, got %r' % e)
+    print('ok, got RuntimeError: %s' % str(e))
 
 # vim: expandtab tabstop=4 shiftwidth=4