BUG: fix output path for WriteImage test

[blowekamp]

Addresses creation of test1.mha file in wrong place.

Fix the variable name and the preprocessor stringification.

PR Checklist

• [ ] No API changes were made (or the changes have been approved)
• [ ] No major design changes were made (or the changes have been approved)
• [ ] Added test (or behavior not changed)
• [ ] Updated API documentation (or API not changed)
• [ ] Added license to new files (if any)
• [ ] Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• [ ] Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for further development details if necessary.

[blowekamp]

What used to be the content of ITK_TEST_OUTPUT_DIR_STR?

It appeared to be undefined and no change directory operation occoured.

[blowekamp]

It seems that when the test is run through ctest the path is automatically changed to the correct TEST_DIR. However when the GTest driver is directly called it is now. The latter may be more consistent with the behavior of the other itk custom test drivers, so this extra change dir may not really be needed.