Adds a unit test for the debugging message triggered when readfile_accel() detects
a non-empty output buffer. The readfile_accel() function directly manipulates the
output buffers, so calling it from within PHPUnit triggers an error like "Test code
or tested code closed output buffers other than its own".
As a workaround, the test runs a CLI script in a separate process, allowing buffer
behaviour to be isolated and debugging output to be captured for assertions.
Trisha Milan