cmd/go: produce a better error for generic test functions
Test functions with type parameters cannot be invoked without type
arguments, so cmd/go would previously fail with a type-checking error in
the test harness.
Avoid this by producing an error explaining that test functions cannot
have type parameters.
Fixes #48953
Change-Id: I4e39c9b7a06c964fad5f12d74d262aa090c25d79
Reviewed-on: https://go-review.googlesource.com/c/go/+/355889
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>