docs: rust: Add rusttest info

Searching the Rust kernel documentation all existing Rust Make
targets (rustavailable, rustfmt, rustfmtcheck, rustdoc and
rust-analyzer) are explicitly documented with their Make commands.
While the Make target rusttest is mentioned two times in the
existing documentation, it's Make command is not explicitly
documented, yet. Add a test section to document this.

While at it, add some info about the more important KUnit testing
too.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20231212081313.226120-1-dirk.behme@de.bosch.com


[ Added "the", newline and quotes for `.config`. Expanded "repos". ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>