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