Add smoke tests

[vasi]

@kevin-vigor can you make sure this does what you need on your systems?

• Make smoke tests run on a wider array of systems:
  • Allow any version of fusermount
  • Test any compressors that overlap between mksquashfs and squashfuse
• Don't depend on hostname for anything, use an envvar
• A bit more care in autoconf-writing, and only running the tests when they might actually complete
• A subset of tests can run on macs, at least if md5sum is installed

[kevin-vigor]

@vasi : ACK, I can confirm this runs properly in my environment and diff looks good to me. Thanks!