Failing tests on windows.

In reference to https://github.com/asdf-vm/asdf/issues/450 and https://github.com/asdf-vm/asdf/pull/451 I have been preparing appveyor CI to run tests. Using cygwin bash most of the tests pass. I need some help in debugging the failing tests and possibly filling up the gap between Windows and Linux platforms.

```
not ok 1 exports ASDF_DIR
# (in test file test/asdf_fish.bats, line 22)
# `")' failed with status 127
# /tmp/bats.493.src: line 22: fish: command not found
not ok 2 adds asdf dirs to PATH
# (in test file test/asdf_fish.bats, line 37)
# `")' failed with status 127
# /tmp/bats.493.src: line 37: fish: command not found
not ok 3 does not add paths to PATH more than once
# (in test file test/asdf_fish.bats, line 54)
# `")' failed with status 127
# /tmp/bats.493.src: line 54: fish: command not found
not ok 4 defines the asdf function
# (in test file test/asdf_fish.bats, line 69)
# `")' failed with status 127
# /tmp/bats.493.src: line 69: fish: command not found
```
```
not ok 69 asdf exec should pass all arguments to executable even if shim is not in PATH
# (in test file test/shim_exec.bats, line 42)
# `[ "$output" == "" ]' failed
```

You can see the logs [here](https://ci.appveyor.com/project/abhishalya/asdf).