Switch from assert_cli to assert_cmd

[AMDmi3]

Test fail with recent rust:

running 1 test
test multiple_bin_fail ... FAILED

failures:

---- multiple_bin_fail stdout ----

thread 'multiple_bin_fail' (2462685) panicked at /home/marakasov/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/assert_cli-0.6.3/src/assert.rs:441:13:
Assertion failed for `cargo run --quiet -- readme --project-root tests/multiple-bin-fail`
with: Unexpected stderr

with: Didn't match.
diff=
 ``+warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/config/badges.rs:181:26
    |
181 | fn percent_encode(input: &str) -> pe::PercentEncode {
    |                          ^^^^     ^^^^^^^^^^^^^^^^^ the same lifetime is hidden here
    |                          |
    |                          the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
181 | fn percent_encode(input: &str) -> pe::PercentEncode<'_> {
    |                                                    ++++

 Error: Multiple binaries found, choose one: [src/entry1.rs, src/entry2.rs]

note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

failures:
multiple_bin_fail

This is caused by two problems
- There is compilation warning (addressed in #114)
- `assert_cli` calls `cargo run` instead of calling a real crate binary, which leads to cargo stderr (which contains a warning) prepended to command output. Since `assert_cli` is deprecated, switch to `assert_cmd` everywhere. Because unlike `assert_cli` it does not ignore leading/trailing whitespace in expected stderr/stdout contents, leading newlines had to be removed in inline expected data. If you find it undesirable, it can be fixed by adding `trim_start` or switching to `indoc` (more convenient multiline string literals) or `insta` (snapshot tests - keeps expected data in separate files and assists in updating them).