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>

tools/test.py

@@ -375,7 +375,10 @@ class TapProgressIndicator(SimpleProgressIndicator):
 
       if output.diagnostic:
         self.severity = 'ok'
-        self.traceback = output.diagnostic
+        if isinstance(output.diagnostic, list):
+          self.traceback = '\n'.join(output.diagnostic)
+        else:
+          self.traceback = output.diagnostic
 
 
     duration = output.test.duration