feat: parallel functional tests (~3x faster CI)

- Split 69 functional tests into 4 groups for parallel execution
- Add check-functional-parallel target to Makefile.am
- Fix VPATH builds with $(srcdir)/tests path
- Add test failure detection verification in CI
- Update Dockerfiles with parallel make -j$(nproc)
- Add --depth 1 for faster git clone in Dockerfiles

tests/functionaltests/proftest.h

@@ -1,8 +1,13 @@
 #ifndef __H_PROFTEST
 #define __H_PROFTEST
 
-#define XDG_CONFIG_HOME "./tests/functionaltests/files/xdg_config_home"
-#define XDG_DATA_HOME   "./tests/functionaltests/files/xdg_data_home"
+/*
+ * XDG paths are now dynamic, generated per-test based on stub_port.
+ * This allows parallel test execution without file conflicts.
+ * Use xdg_config_home and xdg_data_home variables instead of macros.
+ */
+extern char xdg_config_home[256];
+extern char xdg_data_home[256];
 
 extern int stub_port;