tests: portability fixes in tests on amhello examples * tests/amhello-binpkg.test: Don't use "tar xzf too.tag.gz" to extract a gzip-compressed tarball, that's unportable to some tar implementations; use the "gzip -dc fo.tar.gz | tar xf -" idiom instead. * tests/amhello-cflags.test: Likewise. * tests/amhello-cross-compile.test: Likewise. Suggestion from Ralf Wildenhues.