Do not invoke ctest when BUILD_TESTING=OFF

[eramongodb]

Resolve the following error when libmongocrypt is built by the C Driver with CMake 4.4.0 and BUILD_TESTING=OFF:

*********************************
No test configuration file found!
*********************************
Usage
  ctest [options]

Per CMake 4.4.0 release notes:

ctest now exits with non-zero if invoked with no arguments and no test input file exists.

[kevinAlbs]

Thank you!

[eramongodb]

Subtle bugfix: is_true expands the given argument as a variable. Need to pass the name of the boolean variable, not the value.