cmd/go: improve handling of os.DevNull on Windows

The "go test" and "go build" commands have special-case behavior when
passed "-o /dev/null". These checks are case-sensitive and assume that
os.DevNull is an absolute path. Windows filesystems are case-insensitive
and os.DevNull is NUL, which is not an absolute path.

CL 145220 changed filepath.IsAbs to report "NUL" as absolute to work
around this issue; that change is being rolled back and a better fix here
is to compare the value of -o against os.DevNull before attempting to
merge it with a base path. Make that fix.

On Windows, accept any capitilization of "NUL" as the null device.

This change doesn't cover every possible name for the null device, such
as "-o //./NUL", but this test is for efficiency rather than correctness.
Accepting just the most common name is fine.

For #56217.

Change-Id: I60b59b671789fc456074d3c8bc755a74ea8d5765
Reviewed-on: https://go-review.googlesource.com/c/go/+/449117
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/work/build.go

@@ -482,7 +482,7 @@ func runBuild(ctx context.Context, cmd *base.Command, args []string) {
 	pkgs = omitTestOnly(pkgsFilter(pkgs))
 
 	// Special case -o /dev/null by not writing at all.
-	if cfg.BuildO == os.DevNull {
+	if base.IsNull(cfg.BuildO) {
 		cfg.BuildO = ""
 	}