src/test/bench: restructure
Teach compiletest to use multiple --src-base's
Teach the makefile to use multiple src-base's
auto merge of #8458 : cmr/rust/test-restructure, r=brson

This should make benchmarks easier to understand. But, it doesn't work.
BENCH_RS in mk/tests.mk has everything, from what I can tell in remake, but
only those that are direct children of src/test/bench get build and run.
@graydon, can you lend your expertise? I can't make heads or tails of this
makefile.