test(tools/cheddar): Run unit tests during build

TIL `doCheck` is `naersk`'s mechanism for running unit tests during builds.

Change-Id: Ife8eebacdf211ea52ecd50bb7bcdba326db64fbe
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5661
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: wpcarro <wpcarro@gmail.com>

tools/cheddar/default.nix

@@ -3,6 +3,7 @@
 depot.third_party.naersk.buildPackage {
   src = ./.;
   doDoc = false;
+  doCheck = true;
 
   override = x: {
     # Use our custom bat syntax set, which is everything from upstream,