cmd/go: better error for test functions with wrong signature

cmd/go: better error for test functions with wrong signature

Check the function types before compiling the tests. Extend the same
approach taken by the type check used for TestMain function.

To keep existing behavior, wrong arguments for TestMain are ignored
instead of causing an error.

Fixes #14226.

Change-Id: I488a2555cddb273d35c1a8c4645bb5435c9eb91d
Reviewed-on: https://go-review.googlesource.com/19763
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>