HITL test fixes

[martinling]

Fixes some bugs in the HITL test:

• The --save-captures option was broken due to calling File::open rather than File::create.
  • Fixes #158.
• Due to a bug in the SOF checking loop, only SOF packets up to the first bus reset were being counted.
• We were only checking for a small minimum of SOF packets. Calculate what range we should actually expect.
• Handle failures cleanly rather than panicking.
• Ensure that capture is stopped cleanly if test device access fails.