Testsuite: fix debug_rts detection
Running the testsuite without Hadrian should set config.debug_rts correctly too.
Upstreamed from https://github.com/stable-haskell/ghc/pull/42
Running the testsuite without Hadrian should set config.debug_rts correctly too.
Upstreamed from https://github.com/stable-haskell/ghc/pull/42