test.bash: set GOPATH before running go list
test.bash: set GOPATH before running go list

The "go list" command is used only to get the module name.
However, just invoking the command results in a series of network IO
fetching package information about dependencies.
That information is then placed in the default GOPATH,
which previously was the system's default.

This is not generally a problem, but results in additional time spent
in Travis since the system's GOPATH is not cached between test runs.
By setting the GOPATH beforehand to be within TEST_DIR, we can ensure
that the result is cached and preserved between runs.

Change-Id: Ib750a9a31b9891b354fe9a70b3f592d949186ba3
Reviewed-on: https://go-review.googlesource.com/136738
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go