Sanitize dut_id for use in output file name

[jdve]

Sometimes the DUT id may contain characters that aren't appropriate for
file names. Sanitize the dut_id, so that we can safely use any ID.

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[jdve]

@googlebot I signed it!

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[jdve]

@googlebot I signed it!

[lalten]

I've hit an issue before where the filename was too long for the filesystem, this could be guarded against too here.