ci: enable flakybot for test failure detection

[noahdietz]

We should install the flakybot app to notify us of failing tests in our continuous job.

@vishwarajanand has identified a set of already failing tests. Once we do a bit of clean up, we can start the process of enabling the app on the repo. Those tests that aren't addressed initially should result in new issues being created.

[noahdietz]

Based on the docs, since this repo is in googleapis, the app is already installed. We need to make sure the other setup is aligned.

[noahdietz]

It looks like only a few steps are necessary to enable it at this point.

here is where we have sponge_log files being created:

google-cloud-php/.kokoro/continuous/run-tests.sh

Lines 10 to 16 in 8d3c0fd

	mkdir -p ${SHORT_JOB_NAME}/unit
	mkdir -p ${SHORT_JOB_NAME}/snippets
	mkdir -p ${SHORT_JOB_NAME}/system
	UNIT_LOG_FILENAME=${SHORT_JOB_NAME}/unit/sponge_log.xml
	SNIPPETS_LOG_FILENAME=${SHORT_JOB_NAME}/snippets/sponge_log.xml
	SYSTEM_LOG_FILENAME=${SHORT_JOB_NAME}/system/sponge_log.xml

but then they are populated with junit format?

google-cloud-php/.kokoro/continuous/run-tests.sh

Lines 39 to 49 in 8d3c0fd

	echo "Running Unit Test Suite"
	vendor/bin/phpunit -c phpunit${PHPUNIT_SUFFIX}.xml.dist --log-junit ${UNIT_LOG_FILENAME}
	echo "Running Snippet Test Suite"
	vendor/bin/phpunit -c phpunit-snippets.xml.dist --verbose --log-junit \
	${SNIPPETS_LOG_FILENAME}
	echo "Running System Test Suite"
	vendor/bin/phpunit -d memory_limit=512M -c phpunit${PHPUNIT_SUFFIX}-system.xml.dist \
	--verbose --log-junit ${SYSTEM_LOG_FILENAME}

not sure if that needs changing.

@vishwarajanand do you mind handling this? It's already installed on the repo per the documentation

[vishwarajanand]

I've fixed all storage tests (few PRs pending reviews though).
I am looking for Spanner tests to be fixed (most of them have similar failures).
Once thats done, I will look into this.