os: darwin/arm working dir moved for tests

The go_darwin_arm_exec script now tells lldb to move the working
directory into <bundle>/src/os on startup.

Change-Id: I0ada4969e9ea374f08c84ab736aab2097ac73dd8
Reviewed-on: https://go-review.googlesource.com/6369Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

src/os/os_test.go

@@ -59,7 +59,7 @@ var sysdir = func() *sysDir {
 				wd = err.Error()
 			}
 			return &sysDir{
-				wd,
+				filepath.Join(wd, "..", ".."),
 				[]string{
 					"ResourceRules.plist",
 					"Info.plist",