Updated rust_test docs

[UebelAndre]

When using use_libtest_harness users can use --test_arg to run individual tests. I think this functionality should be documented. See below as an example

user@localhost rules_rust % bazel test //tools/runfiles:runfiles_test --test_output=all

INFO: Invocation ID: 96f54232-3330-4ca7-b127-25ee2e44bedd
INFO: Analyzed target //tools/runfiles:runfiles_test (0 packages loaded, 0 targets configured).
INFO: Found 1 test target...
INFO: From Testing //tools/runfiles:runfiles_test:
==================== Test output for //tools/runfiles:runfiles_test:

running 2 tests
test test::test_manifest_based_can_read_data_from_runfiles ... ok
test test::test_can_read_data_from_runfiles ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

================================================================================
Target //tools/runfiles:runfiles_test up-to-date:
  bazel-bin/tools/runfiles/runfiles_test
INFO: Elapsed time: 0.367s, Critical Path: 0.09s
INFO: 2 processes: 1 remote cache hit, 1 darwin-sandbox.
INFO: Build completed successfully, 2 total actions
//tools/runfiles:runfiles_test                                           PASSED in 0.1s

Executed 1 out of 1 test: 1 test passes.
INFO: Build completed successfully, 2 total actions

user@localhost rules_rust % bazel test //tools/runfiles:runfiles_test --test_output=all --test_arg=test::test_manifest_based_can_read_data_from_runfiles

INFO: Invocation ID: 03235fb8-fd9f-4872-bfa3-5d6c56c1821e
INFO: Analyzed target //tools/runfiles:runfiles_test (0 packages loaded, 0 targets configured).
INFO: Found 1 test target...
INFO: From Testing //tools/runfiles:runfiles_test:
==================== Test output for //tools/runfiles:runfiles_test:

running 1 test
test test::test_manifest_based_can_read_data_from_runfiles ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 1 filtered out; finished in 0.00s

================================================================================
Target //tools/runfiles:runfiles_test up-to-date:
  bazel-bin/tools/runfiles/runfiles_test
INFO: Elapsed time: 0.286s, Critical Path: 0.02s
INFO: 2 processes: 2 remote cache hit.
INFO: Build completed successfully, 2 total actions
//tools/runfiles:runfiles_test                                  (cached) PASSED in 0.0s

Executed 0 out of 1 test: 1 test passes.
INFO: Build completed successfully, 2 total actions

closes #417

[UebelAndre]

This documentation is probably a bit limited. Open to suggestions 😄