go/internal/gcimporter: skip TestImportTypeparamTests if GOROOT/test is missing

go/internal/gcimporter: skip TestImportTypeparamTests if GOROOT/test is missing

cmd/distpack removes GOROOT/test, so skip the test if it isn't there.

For #24904.

Change-Id: Iac381517d0540056b2ccea0dc1bd716113b18468
Reviewed-on: https://go-review.googlesource.com/c/go/+/504117
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>