cmd/go: restore TestLegacyMod as a script test

cmd/go: restore TestLegacyMod as a script test

This test was apparently mistakenly removed without a replacement in
CL 213223, but its testdata was left in the tree. This change removes
the orphaned testdata subdirectory, and restores the test that
previously used that data as a self-contained script.

For #27494.

Change-Id: Ice81895a44c558aaab198b8ef7ec046d92f5d58f
Reviewed-on: https://go-review.googlesource.com/c/go/+/417658
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>