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