BUG: Include find GTest CMake code when installed

[blowekamp]

May address: #303

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.