This can easily save hundreds of stat calls per test script,
particularly when testing Go tools.

While here, remove a few uses of the deprecated io/ioutil in cmd,
and check some missed errors in txtar-c.

This will be useful in some cmd/cue test scripts
where we want each test to create a unique remote resource
that includes the current test name as a prefix,
for the sake of more easily seeing which test created which resource.