Update README-devel to suggest testing the entire workspace

The suggestion of running `cargo test` to test "everything" isn't really
accurate anymore, because of the split into multiple crates a while
back.
Let's suggest the usage of `cargo test --workspace` instead, making it
obvious that there is more than just the core library.

Signed-off-by: Daniel Müller <deso@posteo.net>

README-devel.md

@@ -15,7 +15,7 @@ $ cargo check --package blazesym-c --features=generate-c-header
 ## Testing
 All our testing is `cargo` based and a simple
 ```sh
-$ cargo test
+$ cargo test --workspace
 ```
 runs the vast majority of tests. Tests require `sudo` to be set up properly, as
 some of the functionality we rely on is privileged. Test artifacts are