Revert "Teach the makefile to use multiple src-base's"

This reverts commit 43f851d2cb3976655078f032dc1a8cb88f1c8deb.
Revert "Teach compiletest to use multiple --src-base's"

This reverts commit 8a07f5708196dd72ec030018c2a215a4dd823b2e.
Revert "src/test/bench: restructure"

This reverts commit 14cdc26e8a7794e437946f46df5769362b42acdf.
auto merge of #8886 : cmr/rust/test-restructure, r=cmr