Fix CMake trying to pick up test folders outside of the Rerun project/zip (#4770)

### What

This PR changes the `CMakeLists.txt` of `rerun_cpp` to test for the full
path of the `tests` directory to avoid false positives.

Closes #4769 

### Checklist
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* [x] I've included a screenshot or gif (if applicable)
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* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/4770/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/4770/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/4770/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
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usefulness for the next release's CHANGELOG

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Author: KevinGliewe

rerun_cpp/CMakeLists.txt

@@ -186,6 +186,6 @@ install(FILES
 # -----------------------------------------------------------------------------
 # Add tests if they exist (they are not part of the distribution zip).
 # Has direct dependency to arrow, so needs to happen last.
-if(EXISTS tests)
-    add_subdirectory(tests)
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/tests)
+    add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/tests)
 endif()