cmd/go: add a [root] condition to script_test

[root] reports whether the current effective user ID is 0.

Updates #29127

Change-Id: I9ef42f1271ea669689011e7ceff4d918c0cecb6b
Reviewed-on: https://go-review.googlesource.com/c/153637
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/script_test.go

@@ -274,6 +274,8 @@ Script:
 				ok = testenv.HasExternalNetwork()
 			case "link":
 				ok = testenv.HasLink()
+			case "root":
+				ok = os.Geteuid() == 0
 			case "symlink":
 				ok = testenv.HasSymlink()
 			default:

src/cmd/go/testdata/script/README

@@ -75,6 +75,7 @@ should only run when the condition is satisfied. The available conditions are:
    - [cgo], [msan], [race] for whether cgo, msan, and the race detector can be used
    - [net] for whether the external network can be used
    - [link] for testenv.HasLink()
+   - [root] for os.Geteuid() == 0
    - [symlink] for testenv.HasSymlink()
    - [exec:prog] for whether prog is available for execution (found by exec.LookPath)