GN Rules for Rust Tests

The simplest way to build Rust gtest tests is to add them to an existing test binary that already contains tests authored in C++. For example:

test("ui_base_unittests") {
  ...
  sources += [ "my_rust_lib_unittest.rs" ]
  deps += [ ":my_rust_lib" ]
}

Authoring Rust tests in a separate static_library also works, but requires manually declaring the dependency on the support libraries:

rust_static_library("my_rust_lib_unittests") {
  testonly = true
  is_gtest_unittests = true
  crate_root = "my_rust_lib_unittest.rs"
  sources = [ "my_rust_lib_unittest.rs" ]
  deps = [
    ":my_rust_lib",
    "//testing/rust_gtest_interop",
  ]
}
test("ui_base_unittests") {
  ...
  deps += [ ":my_rust_lib_unittests" ]
}