cmd/go/testdata/script: ensure go test -skip skips ExampleTest1

CL 511837 added a check for go test -skip Example but it currently
doesn't verify that the example doesn't show up in the command output.
Add such a check.

For #61482

Change-Id: I3a8f82fc137739bf291f39bf7719ff92cfc74f9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/519595
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: tklauser

src/cmd/go/testdata/script/test_skip.txt

@@ -26,6 +26,7 @@ go test -v -skip Example skip_test.go
 stdout RUN.*Test1
 stdout RUN.*Test2
 stdout RUN.*Test2/3
+! stdout ExampleTest1
 
 -- skip_test.go --
 package skip_test