ci(test): hoist include(GoogleTest) before first gtest_discover_tests (PRE_TEST exit-8 fix)

The plain Linux x86_64 and AsAN+UBSan lanes failed at ctest with exit 8:
  CMake Error at build_*/test/test_dgb_subsidy[1]_include.cmake:6
  Unknown CMake command "gtest_discover_tests_impl".
preceded by "include() given empty file name (ignored)".

include(GoogleTest) sets _GOOGLETEST_DISCOVER_TESTS_SCRIPT (the path to
GoogleTestAddTests.cmake). It was at line ~176, AFTER the first three
gtest_discover_tests() calls (test_dgb_subsidy/coinbase_value/utxo, all
DISCOVERY_MODE PRE_TEST). In PRE_TEST the generated <tgt>[1]_include.cmake
includes that script at ctest time to define gtest_discover_tests_impl; with
the var unset the include path was empty and ctest aborted.

The clean-build-dir step (8d05ef2) removed the cached _tests.cmake that had
masked the broken else-branch, surfacing it on the first PRE_TEST target.
Fix: hoist include(GoogleTest) to the top of the BUILD_TESTING block.

Author: frstrtr

test/CMakeLists.txt

@@ -1,4 +1,13 @@
 if (BUILD_TESTING AND GTest_FOUND)
+    # include(GoogleTest) MUST precede any gtest_discover_tests() call below:
+    # it sets _GOOGLETEST_DISCOVER_TESTS_SCRIPT (path to GoogleTestAddTests.cmake).
+    # In PRE_TEST mode the generated <tgt>[1]_include.cmake includes that script
+    # to define gtest_discover_tests_impl; if the var is unset the include path is
+    # empty and ctest dies "Unknown CMake command gtest_discover_tests_impl" (exit
+    # 8). The clean-build-dir step (8d05ef2b) removed the cached _tests.cmake that
+    # had masked this, surfacing it on test_dgb_subsidy (the first PRE_TEST call).
+    include(GoogleTest)
+
     # DGB block subsidy oracle-conformance (card #156). config_coin.hpp
     # subsidy() is header-only/static; no dgb runtime link required.
     add_executable(test_dgb_subsidy test_dgb_subsidy.cpp)