built-tests: use custom crate to test implicit optional-dependency features aren't there

[syphar]

I extracted the actual error from CI logs:

Documenting stylish-core v0.1.2 (/opt/rustwide/workdir)
: feature has been removed
   --> src/lib.rs:10:38
    |
 10 | #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
    |                                      ^^^^^^^^^^^^ feature has been removed
    |
    = note: removed in CURRENT_RUSTC_VERSION; see <https://github.com/rust-lang/rust/pull/138907> for more information
    = note: merged into `doc_cfg`
 
 error: Compilation failed, aborting rustdoc
 
 For more information about this error, try `rustc --explain E0557`.
 error: could not document `stylish-core`

So it's not fixed in the version yet.

So I added a custom crate that only contains what we want to test.

please validate my assumption for the test case, I'm basing my test just on the name of the test.

[syphar]

I created a PR to the library itself: Nullus157/stylish-rs#38

[syphar]

@GuillaumeGomez I migrated our code to use a custom local crate for testing.

Can you confirm my thought about what case the test should cover, and if the test-crate correctly covers it?