Resolves CXX-3288. Split from #1403.

This PR delays the call to include(FetchMongoC) until after all top-level cache variables have been initialized (notably, including the ENABLE_TESTS option) to avoid the C Driver's own cache variable initialization taking priority over the C++ Driver when conditionally importing the auto-downloaded C Driver via add_subdirectory().

The auto-downloaded C Driver is observed to overwrite the default value (OFF) of the C++ Driver's ENABLE_TESTS cache variable with its own default (ON):

$ cmake -B local-build -LA
...
-- Required MongoDB C Driver libraries not found via find_package()
-- MongoDB C Driver library sources will be downloaded from GitHub
-- Downloading and configuring MongoDB C Driver 2.0.0...
...
-- Downloading Catch2... (!?)
...
-- Downloading Catch2... done.
...
ENABLE_TESTS:BOOL=ON (!?)

According to CMake documentation, set() shouldn't overwrite an existing cache variable:

Since cache entries are meant to provide user-settable values this does not overwrite existing cache entries by default. Use the FORCE option to overwrite existing entries.

The C++ Driver defines ENABLE_TESTS via option(), which creates a cache variable:

In CMake project mode, a boolean cache variable is created with the option value.

However, the C++ Driver does so long after the C Driver has already been imported via include(FetchMongoC). This means the C Driver's ENABLE_TESTS cache variable initialization via set(CACHE) has priority over the C++ Driver's call to option() which is therefore ignored:

25  | option(BUILD_TESTING "When ENABLE_TESTS=ON, include test targets in the \"all\" target")
    .
52  | include(FetchMongoC) # Initializes the ENABLE_TESTS cache variable with "ON".
    .
313 | option(ENABLE_TESTS "Enable building test targets." OFF) # Too late!
    .
320 | if(ENABLE_TESTS)
321 |     enable_testing()
322 | endif()
323 |
324 | add_subdirectory(src) # Sees ENABLE_TESTS=ON -> calls include(FetchCatch2).

To fix this bug, this PR moves the include(FetchMongoC) to come after option(ENABLE_TESTS) (and other top-level cache variables) and just before add_subdirectory(src) (where C Driver targets are required).