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.ok b/selftest/trial_test.ok
index 6ad39a9..8c6a567 100644
--- a/selftest/trial_test.ok
+++ b/selftest/trial_test.ok
@@ -11,6 +11,6 @@
 None
 - test checksum verification
 - detect wrong checksum
-ok, got RuntimeError("Checksum mismatch for '[PATH]/trial_test/invalid_checksum/file2' vs. '[PATH]/trial_test/invalid_checksum/checksums.md5' line 2",)
+ok, got RuntimeError: Checksum mismatch for '[PATH]/trial_test/invalid_checksum/file2' vs. '[PATH]/trial_test/invalid_checksum/checksums.md5' line 2
 - detect missing file
-ok, got RuntimeError("File listed in checksums file but missing in trials dir: '[PATH]/trial_test/missing_file/file2' vs. '[PATH]/trial_test/missing_file/checksums.md5' line 2",)
+ok, got RuntimeError: File listed in checksums file but missing in trials dir: '[PATH]/trial_test/missing_file/file2' vs. '[PATH]/trial_test/missing_file/checksums.md5' line 2