spvc testing: add logging. (#583)
spvc testing: add logging. (#583)

Add some flags to the spvc test script to aid development:
  --log for logging all commands executed,
  --dry-run to not execute commands,
  --give-up to exit after first failure.

One use of the log is to compare behavior to the SPIRV-Cross test,
so we log what SPIRV-Cross would do, even commands we don't need to do.
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