Increased memory usage for PHPUnit

[mikey179]

I just noted that since merging #212 memory usage for running PHPUnit has increased from around 20 MB per run to ~1GB per run (fluctuating between several 100 MB and 2 GB), both with coverage enabled. Disabling coverage doesn't make any difference. I don't know where this comes from but this should definitely be investigated.

[mikey179]

Found it: the problem is the usage of fread($fh, rand()); in vfsStreamWrapperErroneousFileTestCase where rand() often produces large numbers for which PHP has to allocate memory - PHP doesn't know yet that the read will fail and it actually doesn't need that much memory, but it has to allocate beforehand. That also explains why memory usage is fluctuating that much.