Use an anonymous module to test jsinterop_base

This ensures that we'll be testing the targets as if we were a consumer of the module. i.e. we won't be using J2CL from head.

I removed building of the testlib since it doesn't serve any purpose here and would require pulling in dev dependencies.

PiperOrigin-RevId: 788965006

Author: kevinoconnor7

.bcr/presubmit.yml

@@ -1,5 +1,4 @@
 bcr_test_module:
-  module_path: "."
   matrix:
     platform:
       - macos
@@ -11,6 +10,4 @@ bcr_test_module:
       platform: ${{ platform }}
       bazel: ${{ bazel }}
       build_targets:
-        - "//java/..."
-        - "//javatests/jsinterop/base:testlib"
-
+        - "@jsinterop_base//java/..."