tests: relax a test on amhello examples to cater to Solaris tar

tests: relax a test on amhello examples to cater to Solaris tar

* tests/amhello-binpkg.test: When the tar implementation in use
is not GNU tar, relax the tests on tar output, to avoid spurious
failures.  For example, "tar cvf ..." with GNU tar can output
lines like "./usr/bin/hello" on the standard output, while with
Solaris tar it can output lines like "a ./usr/bin/hello 8K" on
standard output, and with Heirloom tar it can output lines like
"a ./usr/bin/hello 15 tape blocks" on standard error.