test: discover gtest tests at ctest startup, not POST_BUILD

CMake 4.4.0's generated discovery scripts omit TEST_TARGET, so every
target's POST_BUILD discovery shares one JSON file
(cmake_test_discovery_e3b0c44298.json, sha256 of the empty string).
Ninja links test binaries in parallel and their discovery steps race on
that file: one truncates it while another parses, producing the
intermittent macOS failure

  ParseTestList.cmake:96: string sub-command JSON failed parsing json
  string ... Line 1, Column 1: Syntax error

Fixed upstream in cmake 4.4.1, but Homebrew CI runners ship 4.4.0.
PRE_TEST discovery runs serially at ctest startup, immune to the race
on any cmake version. Measured cost: 0.8s to enumerate all 772 tests,
cached thereafter (0.12s); build step drops its ~28 post-link discovery
executions.

Author: gmarzot

test/CMakeLists.txt

@@ -1,6 +1,11 @@
 find_package(GTest REQUIRED CONFIG)
 include(GoogleTest)
 
+# Discover tests at ctest startup (serial) instead of POST_BUILD (parallel).
+# Parallel POST_BUILD discovery races on a shared JSON file under cmake 4.4.0,
+# intermittently failing macOS builds with "JSON failed parsing".
+set(CMAKE_GTEST_DISCOVER_TESTS_DISCOVERY_MODE PRE_TEST)
+
 add_library(moqx_test_utils STATIC
   TestUtils.cpp
 )