Simplify, and replace use of QRegExp with QRegularExpression for Qt6
compatibility.
Also fix the test running on Windows. RockboxInfo constructs the
filename from path and filename, so we cannot pass an empty path, since
that results in an invalid path. On Linux / MacOS this works only
because we use an absolute path.
Change-Id: Ieaf30a2df005291d3e997aabf42d64ee832381c2
