PR go/89168
libgo: change gotest to run examples with output
Change the gotest script to act like "go test" and run examples that
have "output" comments. This is not done with full generality, but
just enough to run the libgo tests. Other packages should be tested
with "go test" as usual.
While we're here clean up some old bits of gotest, and only run
TestXXX functions that are actually in *_test.go files. The latter
change should fix https://gcc.gnu.org/PR89168.
Reviewed-on: https://go-review.googlesource.com/c/162139
From-SVN: r268922
