tools: fix type mismatch in test runner

`output.diagnostic` is a list that is appended to on SmartOS when
retrying a test due to `ECONNREFUSED`. The test runner checks if
`output.diagnostic` is truthy and, if so, assigns its value to
`self.traceback`. 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'

PR-URL: https://github.com/nodejs/node/pull/38289


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>