This is necessary for the following:
- to make sure os/exec can be imported
- to make sure internal/testenv can be imported
The internal/testenv package (which imports os/exec) is used by a lot of
tests. By adding support for it, more tests can be run.
This commit adds a bunch of new packages that now pass all tests.
os.Getenv() was already stubbed out, but os.LookupEnv() wasn't. This
will allow me to compile my program unmodified without using separate
files and build tags.
