Update bcr_test_module test matrix (#56)

.bcr/presubmit.yml

diff --git a/.bcr/presubmit.yml b/.bcr/presubmit.yml
index 720e4db..a3de324 100644
--- a/.bcr/presubmit.yml
+++ b/.bcr/presubmit.yml
@@ -11,6 +11,8 @@
 
 bcr_test_module:
   module_path: ""
+  matrix:
+    platform: ["macos", "ubuntu2004"]
   tasks:
     run_test_module:
       name: "Run test module"