Fix Clippy errors in latest Rust

[edmorley]

Fixes Clippy errors like:

warning: unnecessary hashes around raw string literal
   --> libcnb-test/src/macros.rs:19:17
    |
19  | /                 r#"assertion failed: `(left contains right)`
20  | | left (unescaped):
21  | | {}
...   |
24  | | right: `{:?}`"#,
    | |_______________^
...
258 |           assert_contains!("Hello World!", "World");
    |           ----------------------------------------- in this macro invocation
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_raw_string_hashes
    = note: `-W clippy::needless-raw-string-hashes` implied by `-W clippy::pedantic`
    = help: to override `-W clippy::pedantic` add `#[allow(clippy::needless_raw_string_hashes)]`
    = note: this warning originates in the macro `assert_contains` (in Nightly builds, run with -Z macro-backtrace for more info)
help: remove all the hashes around the string literal
    |
19  ~                 r"assertion failed: `(left contains right)`
20  | left (unescaped):
...
23  | left (escaped): `{:?}`
24  ~ right: `{:?}`",

warning: the following explicit lifetimes could be elided: 'a
  --> libcnb-test/src/test_context.rs:27:6
   |
27 | impl<'a> TestContext<'a> {
   |      ^^              ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
   |
27 - impl<'a> TestContext<'a> {
27 + impl TestContext<'_> {

warning: this `map_or` is redundant
   --> libcnb-package/src/package_descriptor.rs:113:13
    |
113 | /             uri.scheme()
114 | |                 .map_or(false, |scheme| scheme.as_str() == "libcnb")
    | |____________________________________________________________________^ help: use is_some_and instead: `uri.scheme().is_some_and(|scheme| scheme.as_str() == "libcnb")`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_map_or
    = note: `#[warn(clippy::unnecessary_map_or)]` on by default

GUS-W-17399229.