tools: fix type mismatch in test runner

[richardlau]

output.diagnostic is a list that is appended to on SmartOS when retrying a test due to ECONNREFUSED.

node/tools/test.py

Line 612 in 2853b76

self.diagnostic = []

node/tools/test.py

Lines 197 to 204 in 2853b76

	# SmartOS has a bug that causes unexpected ECONNREFUSED errors.
	# See https://smartos.org/bugview/OS-2767
	# If ECONNREFUSED on SmartOS, retry the test one time.
	if (output.UnexpectedOutput() and
	sys.platform == 'sunos5' and
	'ECONNREFUSED' in output.output.stderr):
	output = case.Run()
	output.diagnostic.append('ECONNREFUSED received, test retried')

The test runner checks if output.diagnostic is truthy and, if so, assigns its value to self.traceback.

node/tools/test.py

Lines 376 to 378 in 2853b76

	if output.diagnostic:
	self.severity = 'ok'
	self.traceback = output.diagnostic

However self.traceback is supposed to be a string, and _printDiagnostic() in the TapProgressIndicator attempts to call splitlines() on it, which fails if it is a list with:

AttributeError: 'list' object has no attribute 'splitlines'

node/tools/test.py

Lines 318 to 324 in 2853b76

	def _printDiagnostic(self):
	logger.info(' severity: %s', self.severity)
	self.exitcode and logger.info(' exitcode: %s', self.exitcode)
	logger.info(' stack: |-')
	for l in self.traceback.splitlines():
	logger.info(' ' + l)

Seen in https://ci.nodejs.org/job/node-test-commit-smartos/nodes=smartos18-64/38562/console

23:20:25 ok 2976 pummel/test-net-connect-econnrefused
23:20:33   ---
23:20:33   duration_ms: 8.617
23:20:33   severity: ok
23:20:33   stack: |-
23:20:33 Traceback (most recent call last):
23:20:33   File "tools/test.py", line 1753, in <module>
23:20:33     sys.exit(Main())
23:20:33   File "tools/test.py", line 1729, in Main
23:20:33     if RunTestCases(cases_to_run, options.progress, options.j, options.flaky_tests):
23:20:33   File "tools/test.py", line 951, in RunTestCases
23:20:33     return progress.Run(tasks)
23:20:33   File "tools/test.py", line 160, in Run
23:20:33     self.RunSingle(False, 0)
23:20:33   File "tools/test.py", line 221, in RunSingle
23:20:33     self.HasRun(output)
23:20:33   File "tools/test.py", line 395, in HasRun
23:20:33     self._printDiagnostic()
23:20:33   File "tools/test.py", line 323, in _printDiagnostic
23:20:33     for l in self.traceback.splitlines():
23:20:33 AttributeError: 'list' object has no attribute 'splitlines'
23:20:33 make[1]: *** [Makefile:516: test-ci] Error 1

cc @nodejs/python