updated testSoxConversion.js
changed all tests to use assertExactSize instead of assertSize
deleted unused assert functions (assertMd5 and assertSize)
deleted hashing tests (what was the idea behind those?)
deleted '- short' and '- long' labels from test names
fixed modified file sizes AND deleted long test timeout values
    (see commit: `Shortened audio files`)
added test to make sure temp files were cleaned up
    (see commit: `Added cleanup function to delete the temp file afterwards`)
added short timeout in case of error in the 'adjusted volume' test