Add actions for building and testing (#8)

* Add a Github action for building and testing

On Windows, -- -//third_party/... doesn't seem to work, so add all test directories manually. Also run the tests_*. We run only fastbuild tests here, since the opt tests will be run in the release workflow.

Also fix a number of compilation and test issues found along the way.

manifest/BUILD

@@ -119,6 +119,8 @@ cc_library(
         "manifest_updater.h",
         "pending_assets_queue.h",
     ],
+    # Tests don't work under Linux, but we only need it on Windows, anyway.
+    target_compatible_with = ["@platforms//os:windows"],
     deps = [
         ":file_chunk_map",
         ":manifest_builder",