Bad escaping of --test-options

`dhall` has a tasty test-suite which I would like to execute with the arguments `-p /type-inference\/failure/`.

The arguments that `cabal` ends up passing to the `tasty` executable are `-p ''\''/type-inference\/failure/'\'''`:
```
$ cabal test dhall:tasty --test-options "-p '/type-inference\/failure/'" -v
...
Test suite tasty: RUNNING...
/home/simon/src/dhall-haskell/dist-newstyle/build/x86_64-linux/ghc-8.6.5/dhall-1.26.0/t/tasty/noopt/build/tasty/tasty -p ''\''/type-inference\/failure/'\'''
option -p: Could not parse pattern
```