Try to resolve an intermitted crash during coverage collection (#20506)

* Try to resolve an intermitted crash during coverage collection

The only theory I can come up with is that maybe the test completes
before we finish processing the standard input, so I made the test
harness wait for the observatory URL before considering whether the
test has finished or not.

Also, some code cleanup while I'm at it, e.g. avoiding using "onFoo"
for the names of methods, avoiding back-to-back switch statements with
the same values, avoiding `_` argument names, and using `?.` instead
of `if (foo != null) foo.`.

* Revert back the signature of _pipeStandardStreamsToConsole

* Also remove the other additions to this method.