Add test for quoted Env variables passed with BINDGEN_EXTRA_CLANG_ARGS.

Before the test was too broad and just tested for the definition of the variable.
With this test we can ensure that quoting was indeed managed.

Signed-off-by: Axel Viala <axel.viala@darnuria.eu>

bindgen-tests/tests/tests.rs

@@ -390,7 +390,7 @@ fn test_clang_env_args() {
             "test.hpp",
             "#ifdef _ENV_ONE\nextern const int x[] = { 42 };\n#endif\n\
              #ifdef _ENV_TWO\nextern const int y[] = { 42 };\n#endif\n\
-             #ifdef NOT_THREE\nextern const int z[] = { 42 };\n#endif\n",
+             #if defined NOT_THREE && NOT_THREE == 1\nextern const int z[] = { 42 };\n#endif\n",
         )
         .generate()
         .unwrap()