* Adds local file:// URL support to the [validator_list_sites] stanza.
The file:// URL must not contain a hostname. Allows a rippled node
operator to "sideload" a new list if their node is unable to reach
a validator list's web site before an old list expires. Lists
loaded from a file will be validated in the same way a downloaded
list is validated.
* Generalize file/dir "guards" from Config test so they can be reused
in other tests.
* Check for error when reading validators.txt. Saves some parsing and
checking of an empty string, and will give a more meaningful error.
* Completes RIPD-1674.
Fixes: RIPD-1575. Fix argument passing to runner. Allow multiple unit
test selectors to be passed via --unittest argument. Add optional
integer priority value to test suite list. Fix several failing manual
tests. Update CLI usage message to make it clearer.
When a test suite starts and ends, it informs the parent process. If the parent
has received a start message without a matching end message it reports that a
child may have crashed in that suite.
