fix: macOS E2E — don't detect Apple Clang as gcc capability

macOS g++ is Apple Clang, not real GCC. Tests requiring gcc need
GNU GCC for module-specific behavior (gcm.cache, etc.). Also fix
27_self_contained_home tag to elf (assumes Linux sandbox layout).

Author: Sunrisepeak

tests/e2e/27_self_contained_home.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# requires: unix-shell
+# requires: elf
 # 27_self_contained_home.sh — verifies mcpp's self-contained home behaviour.
 #
 # Without MCPP_HOME set, mcpp resolves its home from the binary's location:

tests/e2e/run_all.sh

@@ -59,7 +59,8 @@ case "$OS" in
         ;;
     Darwin)
         CAPS+=(unix-shell)
-        command -v g++      &>/dev/null && CAPS+=(gcc)
+        # macOS g++ is Apple Clang, not real GCC — don't add gcc capability.
+        # Tests requiring gcc need actual GNU GCC (modules, gcm.cache, etc.)
         ;;
     MINGW* | MSYS* | CYGWIN*)
         # Git Bash / MSYS2 on Windows: symlinks need admin or Developer Mode