Rename the TESTING_BUILD macro to be _azure_TESTING_BUILD to highlight this is internal only and reduce chances of collision.

[ahsonkhan]

Fixes #5334

[antkmsft]

Please remove any mentions of BUILD_TESTING cmake flag in our docs and cmake options. I've found two:

1. azure-sdk-for-cpp/CONTRIBUTING.md

   Lines 135 to 137 in 93d8c12

   	<td>BUILD_TESTING</td>
   	<td>Generates Unit Test for compilation. CMake will automatically download and build g-test.<br>After Compiling, use `ctest` to run Unit Test.</td>
   	<td>OFF</td>

2. azure-sdk-for-cpp/cmake-modules/FolderList.cmake

   Line 55 in 93d8c12

   option(BUILD_TESTING "Build test cases" OFF)

   
   There probably should be a paragraph saying that when the project is build with unit tests, those SDK binaries that you get as a result of such build, should not be used by customer's code. They are for unit tests only.

[ahsonkhan]

Please remove any mentions of BUILD_TESTING cmake flag in our docs and cmake options.

BUILD_TESTING is still a valid cmake flag that is intended for use by contributors to the repo. It is also used in our CI pipelines for legs that build tests and ones that only build the source. It isn't meant for end users of the SDK, but is certainly needed by and intended for folks who build, test, and contribute to this OSS project.

I would not want to remove it from the contribution guide because then no new contribute or member of the project would know where to go to figure out steps to build the tests. The contributing guide is for that exact purpose.

Instead, as part of #5335, I will add a note to clarify it isn't meant to be used in production when consuming the SDK, and only meant for contributors to the repo.

There probably should be a paragraph saying that when the project is build with unit tests, those SDK binaries that you get as a result of such build, should not be used by customer's code. They are for unit tests only.

Agreed.

[antkmsft]

Thanks, I think that'd work too.