feat(tests): add cli test for custom fn attributes

Author: rich-ayr

bindgen-tests/tests/expectations/tests/attribute-custom-cli.rs

@@ -1,4 +1,4 @@
-// bindgen-flags: --default-enum-style rust --default-non-copy-union-style manually_drop --no-default=".*" --no-hash=".*" --no-partialeq=".*" --no-debug=".*" --no-copy=".*" --with-attribute-custom="foo_[^e].*=#[doc(hidden)]" --with-attribute-custom-struct="foo.*=#[derive(Default)]" --with-attribute-custom-enum="foo.*=#[cfg_attr(test, derive(PartialOrd, Copy))]" --with-attribute-custom-union="foo.*=#[derive(Clone)],#[derive(Copy)]"
+// bindgen-flags: --default-enum-style rust --default-non-copy-union-style manually_drop --no-default=".*" --no-hash=".*" --no-partialeq=".*" --no-debug=".*" --no-copy=".*" --with-attribute-custom="foo_[^e].*=#[doc(hidden)]" --with-attribute-custom-struct="foo.*=#[derive(Default)]" --with-attribute-custom-enum="foo.*=#[cfg_attr(test, derive(PartialOrd, Copy))]" --with-attribute-custom-union="foo.*=#[derive(Clone)],#[derive(Copy)]" --with-attribute-custom-function="foo.*=#[must_use],#[doc(hidden)]"
 struct foo_struct {
     int inner;
 };
@@ -12,3 +12,4 @@ union foo_union {
 struct non_matching {
     int inner;
 };
+int foo_function() { return 1; }