Fix -test.run test selection with gotestmain

matchString was implemented separately on older go versions since they
didn't want a build dependency on the regex package from the testing
package.

Starting in Go 1.8, this is implemented using a internal package to
break the dependency, but until we require that everywhere, just copy
the regex implementation like the old `go test` did. I'm not sure we'll
be able to use the internal package in the future anyways.

Change-Id: Ief37542640026f9d73a75fd802f2691af50e2511