Expand the test directory to real path

When the test source directory path contains symbolic links, that
directory might be placed earlier in the result of
`IRB::InputCompletor.retrieve_gem_and_system_load_path`, than other
directories in the source tree that are expanded to the real path.  In
that case, the test file in "csv" under the test directory of csv gem
instead of the library path will be the first candidate, in
`TestIRB::TestCompletion#test_complete_require_library_name_first`.

```
<"'csv"> expected but was
<"'csv/helper">.
```

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