cmd/dist: use registerStdTestSpecially for normal Go tests only

It was my oversight in CL 463276 to skip registerStdTestSpecially
packages in the race bench test registration loop. Package testdir
has no benchmarks and doesn't need to be skipped. (And if it had
benchmarks, it's very unlikely they'd need any special handling.)

By now there are more cmd/cgo/internal/... packages that are registered
specially, and there isn't a need for their few benchmarks not to be
used for the purpose of race bench tests. If the 3 benchmarks in
cmd/cgo/internal/test were to require something special, then we can
add it to a new registerRaceBenchTestSpecially map with a comment, and
do register them specially in registerTests instead of forgetting to.

This restores the automatic 'go_test_bench:cmd/cgo/internal/test'
registration and reduces prevalence of registerStdTestSpecially a bit.

For #37486.
For #56844.

Change-Id: I1791fe5bf94cb4b4e0859c5fff4e7a3d5a23723e
Reviewed-on: https://go-review.googlesource.com/c/go/+/494656
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>

Author: dmitshur

src/cmd/dist/test.go

@@ -584,9 +584,6 @@ func (t *tester) registerTests() {
 		}
 		if t.race {
 			for _, pkg := range pkgs {
-				if registerStdTestSpecially[pkg] {
-					continue
-				}
 				if t.packageHasBenchmarks(pkg) {
 					t.registerRaceBenchTest(pkg)
 				}