Function to recursively check all detected files for a specific string

[jfy133]

Currently there are two helper methods for checking output files:

• Find all files with a pattern and generate an md5 (when contents stable between tests)
• Find all files with a pattern and check existence by file name (when contents are unstable between tests)

While the of unstability fallback is good e.g. binary files, it's not as good as checking inside a file for consistent contents (E.g. when maybe it's only a single timestamp inside the file is unstable).

An intermediate check is to check for certain strings, e.g. maybe 'SUCCESSFUL RUN' at the end of a log.

It would be nice to have a function that does the same as the two existing methods, but with the string checking functionality.