Carefully select a default that will do the right thing when inside a
module, as well as when building ad-hoc packages.
This means we no longer need to look at the compiler's -std flag, which
is nice.
Also replace foo.com/ with test/, as per golang/go#37641.
Fixes#7.
The problem with the "grep" built-in command is that it prints the
entire data if there is an error. We don't want megabytes of binary
output for a test.
Daniel Martí