Add flaky decorator to long_callback tests to retry up to 3 times

[jonmmease]

🩹 to retry the long_callback tests up to 3 times. A ===Flaky Test Report=== section should be printed out in the test logs that reports whether tests had to be rerun.

[jonmmease]

Here's an example of the flaky test support that gets printed after the warnings (in this case nothing needed to be rerun)

===Flaky Test Report===

test_lcbc001_fast_input[diskcache] passed 1 out of the required 1 times. Success!
test_lcbc001_fast_input[celery] passed 1 out of the required 1 times. Success!
test_lcbc002_long_callback_running[diskcache] passed 1 out of the required 1 times. Success!
test_lcbc002_long_callback_running[celery] passed 1 out of the required 1 times. Success!
test_lcbc003_long_callback_running_cancel[diskcache] passed 1 out of the required 1 times. Success!
test_lcbc003_long_callback_running_cancel[celery] passed 1 out of the required 1 times. Success!
test_lcbc004_long_callback_progress[diskcache] passed 1 out of the required 1 times. Success!
test_lcbc004_long_callback_progress[celery] passed 1 out of the required 1 times. Success!
test_lcbc005_long_callback_caching[diskcache] passed 1 out of the required 1 times. Success!
test_lcbc005_long_callback_caching[celery] passed 1 out of the required 1 times. Success!
test_lcbc006_long_callback_caching_multi[diskcache] passed 1 out of the required 1 times. Success!
test_lcbc006_long_callback_caching_multi[celery] passed 1 out of the required 1 times. Success!
test_lcbc007_validation_layout[diskcache] passed 1 out of the required 1 times. Success!
test_lcbc007_validation_layout[celery] passed 1 out of the required 1 times. Success!

===End Flaky Test Report===

[jonmmease]

There's an unrelated percy dash table test timeout, but everything else went through

[alexcjohnson]

💃 works for me!