cmake: Clean up cases when to invoke enable_testing()

The `enable_testing()` command invocation is required for `add_test()`
commands, which are used only for `{noverify_}tests`, `exhaustive_tests`
and examples.

CMakeLists.txt

@@ -269,7 +269,7 @@ if(SECP256K1_BUILD_CTIME_TESTS)
   unset(msan_enabled)
 endif()
 
-if(SECP256K1_BUILD_BENCHMARK OR SECP256K1_BUILD_TESTS OR SECP256K1_BUILD_EXHAUSTIVE_TESTS OR SECP256K1_BUILD_CTIME_TESTS OR SECP256K1_BUILD_EXAMPLES)
+if(SECP256K1_BUILD_TESTS OR SECP256K1_BUILD_EXHAUSTIVE_TESTS OR SECP256K1_BUILD_EXAMPLES)
   enable_testing()
 endif()