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
Go 1.6 fixed a bug where compiling with the -pack flag allowed non-main
packages to link.
https://github.com/golang/go/issues/13468
This change uses package main for choosestage and gotestmain.
Users that want to enable this option can use the '-t' option to
bootstrap.bash when passing '-r'. Builders that want to enable this can
set the RUN_TESTS environment variable in their bootstrap.bash.
The gotestmain tools is needed to write the main functions for the test
binaries, since 'go test' doesn't work well in this environment.
Change-Id: Iec5c2b5c9c3f5e3ba0ac8677fb88f5e963f9bd3f
