Add assert_contains! macro

[Malax]

Add assert_contains! macro for asserting patterns in strings. Useful for libcnb-test users that commonly want to match the output of a pack run.

RustDoc

Asserts that left contains right.

Commonly used when asserting pack output in integration tests. Expands to a str::contains
call and logs left (in unescaped and escaped form) as well as right on failure.

Example

let output = "Hello World!\nHello Integration Test!";
assert_contains!(output, "Integration");

Closes GUS-W-10701312

[edmorley]

Thank you for adding this! I suspect we'll be using it a lot :-)