share one loadTestConfig() helper across the tests

19 test translation units each defined an identical local loadConfig().
They now include src/test/TestConfig.h, which lives off the lib/ui/app
include path like SimulationTestAccess.h.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GH8ZMRY3vhxxXcaUxBqxkk

src/test/CMakeLists.txt

@@ -2,6 +2,8 @@
 add_files(
     TEST_FILES
 
+    TestConfig.h
+
     test.cpp
     FormulaTest.cpp
     ConfigLoaderTest.cpp