Version 6 fails when BUILD_TESTING=OFF

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/210)) by yurivict (Bitbucket: [yurivict](https://bitbucket.org/%7B7012cf74-23b1-4689-8f6d-63be51ab0d44%7D/)).**

----------------------------------------
```
/usr/ports/misc/sdformat/work-6/osrf-sdformat-a76ccb633a52bcf87b169f7dcbaeac3c4623d681/src/ign_TEST.cc:24:10: fatal error: 'test_config.h' file not found
#include "test_config.h"
^~~~~~~~~~~~~~~
```

rev. a76ccb633a52bcf87b169f7dcbaeac3c4623d681

Start with src/ign_TEST.cc and its test_config.h include, then inspect the build configuration for BUILD_TESTING=OFF. Reproduce the reported failure with testing disabled and verify that the build completes without requiring test_config.h; confirm that the testing-enabled build still works.

Written by the indexing model from the issue text.