test: skip when executing on an unsupported arch

[imjasonh]

If you melange build a package for one arch, it should not cause an error to melange test that package on a different arch -- it shouldn't even care that the package to test doesn't exist.

Tested with:

$ go run ./ build examples/one-arch.yaml
# succeeds
$ go run ./ test examples/one-arch.yaml  ./packages/x86_64/one-arch-0.0.1-r0.apk --arch=x86_64
# passes              
$ go run ./ test examples/one-arch.yaml  ./packages/x86_64/one-arch-0.0.1-r0.apk --arch=aarch64              
⚠️            | skipping test for one-arch on arm64
$ go run ./ test examples/one-arch.yaml  ./packages/armv7/one-arch-0.0.1-r0.apk --arch=aarch64
⚠️            | skipping test for one-arch on arm64

[vaikas]

Thank you!