Fix for t/casing.t failure on mac os

The following test fails on mac os, where /var is a symlink to /private/var

```
 t/casing.t ............. 1/?
 #   Failed test 'Found definition from second file'
 #   at t/casing.t line 53.
 #          got: '/private/var/folders/zz/zyxvpxvq6csfxvn_n00026dc000jlb/T/5fRT_POw_o/other'
 #     expected: '/var/folders/zz/zyxvpxvq6csfxvn_n00026dc000jlb/T/5fRT_POw_o/other'
 # Looks like you failed 1 test of 19.
 t/casing.t ............. Dubious, test returned 1 (wstat 256, 0x100)
 Failed 1/19 subtests
```

The config filename is converted to an absolute path inside load_file, but the test
checks the original path. This changes the test to use the same call to Cwd::abs_path.