test/run: make GO_GCFLAGS mean same thing it does during make.bash

-gcflags=-flag means apply the flags only to the package named
on the command line (the main package, for these tests).

-gcflags=all=-flag means apply the flags to everything in the build,
including the standard library.

cmd/dist uses -gcflags=all=$GO_GCFLAGS, so test/run should do the same,
as the comment already explains, to avoid rebuilding the entire standard
library without the flags during test/run's builds.

We changed the scope of the flags without a pattern a few releases
ago and missed this one.

Change-Id: I039e60ca619d39e5b502261d4a73e1afc7e3f9fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/213827
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/run.go

@@ -459,7 +459,7 @@ func init() { checkShouldTest() }
 // or else the commands will rebuild any needed packages (like runtime)
 // over and over.
 func goGcflags() string {
-	return "-gcflags=" + os.Getenv("GO_GCFLAGS")
+	return "-gcflags=all=" + os.Getenv("GO_GCFLAGS")
 }
 
 // run runs a test.