Skip more validation

[lukemassa]

What

Right now, the code skips validating the end of the pattern in MatchUnvalidated() if it finds that pattern has not matched. However, if we get to the end of the name and there is still pattern that is not "zero length", we don't need to validate that either.

Also add some unit tests to make sure we are skipping validation when we can.

Why

Speed up the code a bit by skipping more unnecessary validation.

Tests

I added a unit test specifically to see what errors are being detected whether validate is set to true or false.

When those tests are run on main, they show a single error:

    doublestar_test.go:304: #2. Unvalidated error of Match(`a[`, `a`) = syntax error in pattern want <nil>

This is capturing the fact that we got to the end of the name, had matched the whole way, but were unnecessarily doing additional validation instead of immediately returning false.

[bmatcuk]

Looks good, thanks!