Support passing `String` to `assert_contains!` not just `&str`

[edmorley]

With assert_eq! I can do this:

            assert_eq!(
                context.pack_stdout,
                formatdoc! {"
                    FOO
                "}
            );

However, doing the same with assert_contains!, I get:

error[E0277]: expected a `FnMut<(char,)>` closure, found `String`
    --> tests/integration.rs:16:13
     |
16   | /             assert_contains!(
17   | |                 context.pack_stdout,
18   | |                 formatdoc! {"
19   | |                     FOO
20   | |                 "}
21   | |             );
     | |_____________^ expected an `FnMut<(char,)>` closure, found `String`
     |
     = help: the trait `FnMut<(char,)>` is not implemented for `String`
     = help: the following other types implement trait `Pattern<'a>`:
               &'b String
               &'b str
               &'c &'b str
     = note: required because of the requirements on the impl of `Pattern<'_>` for `String`
note: required by a bound in `core::str::<impl str>::contains`
    --> /Users/emorley/.rustup/toolchains/beta-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/str/mod.rs:1054:28
     |
1054 |     pub fn contains<'a, P: Pattern<'a>>(&'a self, pat: P) -> bool {
     |                            ^^^^^^^^^^^ required by this bound in `core::str::<impl str>::contains`
     = note: this error originates in the macro `assert_contains` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.

Instead I have to convert the String to a &str for the formatdoc! argument to assert_contains!:

            assert_contains!(
                context.pack_stdout,
                &formatdoc! {"
                    FOO
                "}
            );

It would be great if assert_contains! supported this without the need to manually convert to a reference.

[edmorley]

xref #477