The existing strided readahead tests are not stringent enough, and pass even though strided readahead is broken.

We have a series of fixes that I believe get strided readahead working 100%, but it's not useful/practical to write tests for each fix individually, since strided readahead is still partly broken until all the fixes are in.

Instead, I'm going to write some generic strided readahead tests and put them at the end of the series.  This will let us verify correct operation with all the patches in place.