CMakeLists.txt: set(RE2_BUILD_TESTING OFF CACHE BOOL "")

This fixes the following build error:

```
The following tests FAILED:
    1 - charclass_test (Not Run)
    2 - compile_test (Not Run)
    3 - filtered_re2_test (Not Run)
    4 - mimics_pcre_test (Not Run)
    ...
```

e.g. https://github.com/firebase/firebase-cpp-sdk/runs/6676610733

This is caused by the addition of re2 to the firebase-ios-sdk in firebase/firebase-ios-sdk#9488.

I'm not sure _why_ these tests can't be found; but they're superfluous anyways. So just disable them.

Author: dconeybe

CMakeLists.txt

@@ -214,6 +214,9 @@ endif()
 # firebase-ios-sdk since it's not needed and can sometimes fail to build.
 set(FIRESTORE_INCLUDE_OBJC OFF CACHE BOOL "Disabled for the CPP SDK")
 
+# Disable re2 build tests
+set(RE2_BUILD_TESTING OFF CACHE BOOL "")
+
 if(FIREBASE_CPP_USE_PRIOR_GRADLE_BUILD)
   # Quote meta characters in ${CMAKE_CURRENT_LIST_DIR} so we can
   # match it in a regex.