run-tests: ignore PermissionError when checking available ports
I'm not sure what this is, but I'm getting it occasionally when running in WSL.
When it was raised, none of the tests could run.
#require rustfmt test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
$ for f in `testrepohg files 'glob:**/*.rs'` ; do
> $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
> done