Standardize test output

[alexreinking]

Our tests (those in /tests/) try to report success via return code 0 and printing a "Success" message. Other tests "succeed" but actually don't run for environmental reasons (e.g. no cuda in HL_TARGET). Detecting these cases should be easy to do by hand.

When a test succeeds, it should print a line containing the string "Success!". When a test fails, it should return non-zero and not print "Success!". When a test chooses not to run, it should print a line beginning with "[SKIP]" and listing a reason for skipping.

In the special case of error tests, they should "succeed" by returning non-zero and "fail" by printing "Success!". The surrounding test infrastructure should reverse the actual condition for these tests.

In the special case of error tests, they should never print "Success!" but should instead print at least one line beginning with "Warning:".

[alexreinking]

This has been implemented as part of the testing improvements in #4644.