GM: add self-test that sets --readPath to a nonexistent directory path

gm/tests/run.sh

 #!/bin/bash
 
 # Self-tests for gm, based on tools/tests/run.sh
 #
 # These tests are run by the Skia_PerCommit_House_Keeping bot at every commit,
 # so make sure that they still pass when you make changes to gm!
 #
 # To generate new baselines when gm behavior changes, run gm/tests/rebaseline.sh
 #
 # TODO: because this is written as a shell script (instead of, say, Python)
@@ skipping 133 matching lines @@
 gm_test "--verbose --hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/images/identical-pixels" "$GM_OUTPUTS/compared-against-identical-pixels-images"
 gm_test "--verbose --hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/json/identical-pixels.json" "$GM_OUTPUTS/compared-against-identical-pixels-json"
 
 # Compare generated image against an input image file with different pixels.
 gm_test "--verbose --hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/images/different-pixels" "$GM_OUTPUTS/compared-against-different-pixels-images"
 gm_test "--verbose --hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/json/different-pixels.json" "$GM_OUTPUTS/compared-against-different-pixels-json"
 
 # Compare generated image against an empty "expected image" dir.
 gm_test "--verbose --hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/images/empty-dir" "$GM_OUTPUTS/compared-against-empty-dir"
 
+# Compare generated image against a nonexistent "expected image" dir.
+gm_test "--verbose --hierarchy --match selftest1 $CONFIGS -r ../path/to/nowhere" "$GM_OUTPUTS/compared-against-nonexistent-dir"
+
 # Compare generated image against an empty "expected image" dir, but NOT in verbose mode.
 gm_test "--hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/images/empty-dir" "$GM_OUTPUTS/nonverbose"
 
 # If run without "-r", the JSON's "actual-results" section should contain
 # actual checksums marked as "failure-ignored", but the "expected-results"
 # section should be empty.
 gm_test "--verbose --hierarchy --match selftest1 $CONFIGS" "$GM_OUTPUTS/no-readpath"
 
 # Test what happens if a subset of the renderModes fail (e.g. pipe)
 gm_test "--simulatePipePlaybackFailure --verbose --hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/json/identical-pixels.json" "$GM_OUTPUTS/pipe-playback-failure"
 
 # Confirm that IntentionallySkipped tests are recorded as such.
 gm_test "--verbose --hierarchy --match selftest1 selftest2 $CONFIGS" "$GM_OUTPUTS/intentionally-skipped-tests"
 
 # Ignore some error types (including ExpectationsMismatch)
 gm_test "--ignoreErrorTypes ExpectationsMismatch NoGpuContext --verbose --hierarchy --match selftest1 $CONFIGS -r $GM_INPUTS/json/different-pixels.json" "$GM_OUTPUTS/ignore-expectations-mismatch"
 
 echo "All tests passed."