cmd/go: more informative test failures when GOROOT is stale

If GOROOT is stale, test fail when commands unexpectedly write to GOROOT.
Include an message in the test failure indicating that this is a possible
and expected reason for the failure, and how to fix it.

For #48698.

Change-Id: I057c20260bab09aebf684e8f20794ab8fc0ede1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/448895
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>

src/cmd/go/go_test.go

@@ -129,7 +129,8 @@ func TestMain(m *testing.M) {
 					}
 					callerPos = fmt.Sprintf("%s:%d: ", file, line)
 				}
-				return fmt.Errorf("%stestgo must not write to GOROOT (installing to %s)", callerPos, filepath.Join("GOROOT", rel))
+				notice := "This error error can occur if GOROOT is stale, in which case rerunning make.bash will fix it."
+				return fmt.Errorf("%stestgo must not write to GOROOT (installing to %s) (%v)", callerPos, filepath.Join("GOROOT", rel), notice)
 			}
 		}