runtests; give each server a unique log lock file

Logs are written by several servers and all of them must be finished
writing before the test results can be determined. This means each
server must have its own lock file rather than sharing a single one,
which is how it was done up to now. Previously, the first server to
complete a test would clear the lock before the other server was done,
which caused flaky tests.

Lock files are now all found in their own directory, so counting locks
equals counting the files in that directory.  The result is that the
proxy logs are now reliably written which actually changes the expected
output for two tests.

Fixes #11231
Closes #11259

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