re-organize samples + integration tests to improve caching

[tstromberg]

This re-organizes tests for speed:

• re-use the cloned samples repo directory for new commits (just swap the commit out)
• re-use previously decompressed files
• remove the test_data file dance

To support this, this PR makes the following changes:

• The out/samples-* folder is now named out/<name of org>/<name of repo>
• The samples/ folder is no longer used
• The test_data directory has been renamed tests
• go test . now works from the newly named tests/ subdirectory
• make test is now split into make integration (slow integration tests) and make test (fast unit tests).