Add test for quoted Env variables passed with BINDGEN_EXTRA_CLANG_ARGS.

[darnuria]

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

Context: I don't know if it's mandatory to check this kind of shell quoting detail, but wandering the code due to the shlex advisory I found this case not covered so I submit a patch!

It's used here:

rg -C 5 shlex
[...[
bindgen/lib.rs
308-        Some(s) => s,
309-    };
310-
311-    // Try to parse it with shell quoting. If we fail, make it one single big argument.
312:    if let Some(strings) = shlex::split(&extra_clang_args) {
313-        return strings;
314-    }
315-    vec![extra_clang_args]
316-}

Edited: has some not commited stuff in the rg cmd