Work around Travis failures on HEAD due to Cabal bug

The previous failures were due to this bug:
haskell/cabal#1938

.travis.yml

@@ -35,6 +35,18 @@ before_script:
      else streaming=streaming
      fi
      export streaming
+ - | # skip cabal test on HEAD due to this bug:
+     # https://github.com/haskell/cabal/issues/1938
+     # this is fine as the test will be run during installation anyway
+     if [ "$CABALVER" = head ]
+     then def_cabal_test='cabal_test() {
+          echo "warning: cabal test skipped due to Cabal bug #1938"
+     }'
+     else def_cabal_test='cabal_test() {
+          cabal test "$@"
+     }'
+     fi
+     export def_cabal_test
  - | # check if '--run-tests' is supported (didn't exist until 1.20);
      # tests must be run prior to install as packages may be broken by install
      if cabal 2>&1 install --run-tests __dummy |
@@ -63,6 +75,6 @@ script:
  - cabal build
  - cabal check
  - cabal sdist
- - cabal test --show-details=$streaming
+ - eval "$def_cabal_test" && cabal_test --show-details=$streaming
  - eval "$def_cabal_install_run_tests" &&
    cabal_install_run_tests --force-reinstalls dist/*-*.tar.gz