Tests select when to use --cfg debug, regardless of configure settings.

Remove directive, if present, from CFG_RUSTC_FLAGS.

r? @huonw 

Fix #7898.

(One alternative tack is to build up distinct CFG_TEST_RUSTC_FLAGS
alongside CFG_RUSTC_FLAGS; but currently debug is the only --cfg flag
ever added to CFG_RUSTC_FLAGS; the other contents of CFG_RUSTC_FLAGS
are a mix of -Z flags and a few other switches like O, which seem to
make sense to propogate to the tests.)