buildRustPackage: Add cargoTestFlags

This makes it possible to pass flags to `cargo test`, which is needed if
a crate is compiled with custom feature flags.

doc/languages-frameworks/rust.section.md

@@ -193,6 +193,13 @@ rustPlatform.buildRustPackage {
 Please note that the code will be compiled twice here: once in `release` mode
 for the `buildPhase`, and again in `debug` mode for the `checkPhase`.
 
+Test flags, e.g., `--features xxx/yyy`, can be passed to `cargo test` via the
+`cargoTestFlags` attribute.
+
+Another attribute, called `checkFlags`, is used to pass arguments to the test
+binary itself, as stated
+(here)[https://doc.rust-lang.org/cargo/commands/cargo-test.html].
+
 #### Tests relying on the structure of the `target/` directory
 
 Some tests may rely on the structure of the `target/` directory. Those tests