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>

Author: dirkbehme

Documentation/rust/general-information.rst

@@ -77,3 +77,27 @@ configuration:
 	#[cfg(CONFIG_X="y")]   // Enabled as a built-in (`y`)
 	#[cfg(CONFIG_X="m")]   // Enabled as a module   (`m`)
 	#[cfg(not(CONFIG_X))]  // Disabled
+
+
+Testing
+-------
+
+There are the tests that come from the examples in the Rust documentation
+and get transformed into KUnit tests. These can be run via KUnit. For example
+via ``kunit_tool`` (``kunit.py``) on the command line::
+
+	./tools/testing/kunit/kunit.py run --make_options LLVM=1 --arch x86_64 --kconfig_add CONFIG_RUST=y
+
+Alternatively, KUnit can run them as kernel built-in at boot. Refer to
+Documentation/dev-tools/kunit/index.rst for the general KUnit documentation
+and Documentation/dev-tools/kunit/architecture.rst for the details of kernel
+built-in vs. command line testing.
+
+Additionally, there are the ``#[test]`` tests. These can be run using
+the ``rusttest`` Make target::
+
+	make LLVM=1 rusttest
+
+This requires the kernel ``.config`` and downloads external repositories.
+It runs the ``#[test]`` tests on the host (currently) and thus is fairly
+limited in what these tests can test.