Add buildscripts/promote_silent_failures.py script to convert silent test failures into non-silent test failures

The buildscripts/promote_silent_failures.py script takes

1. the path to a single report.json file as a command line argument, as well as
2. optionally a path to an output filename via a --output-file command line argument (defaults to stdout)

and writes a report.json file to the specified location.

Note: This is effectively how the command line parsing for the buildscripts/combine_reports.py script works with the exception that the buildscripts/promote_silent_failures.py script should only accept a single report.json file and error if multiple are specified.

For any test files that for at least 2 executions in the report.json file have a "silentfail" status, the buildscripts/promote_silent_failures.py script should change the outputted report to have a "fail" status instead.

Note: The TestReport.from_dict() class method should be used to construct the TestReport instance corresponding to the specified report.json file to avoid manipulating the Python dict structure directly after parsing the JSON file.