refactor to use _is_test attribute instead

This addresses the problem in a different way that allows us to not change the existing logic, but instead differentiate one step further with attributes

Author: Aghassi

js/private/js_binary.bzl

@@ -559,8 +559,10 @@ def _js_binary_impl(ctx):
 
     providers = []
 
-    # We have to instruct test rule implementers to have this attribute present.
-    if ctx.attr.testonly and ctx.configuration.coverage_enabled and hasattr(ctx.attr, "_lcov_merger"):
+    if ctx.attr.testonly and ctx.configuration.coverage_enabled and hasattr(ctx.attr, "_is_test"):
+        # We have to instruct rule implementers to have this attribute present.
+        if not hasattr(ctx.attr, "_lcov_merger"):
+            fail("_lcov_merger attribute is missing and coverage was requested")
         # We have to propagate _lcov_merger runfiles since bazel does not treat _lcov_merger as a proper tool.
         # See: https://github.com/bazelbuild/bazel/issues/4033
         runfiles = runfiles.merge(ctx.attr._lcov_merger[DefaultInfo].default_runfiles)
@@ -638,6 +640,7 @@ the contract between Bazel and a test runner.""",
             default = Label("//js/private/coverage:merger"),
             cfg = "exec",
         ),
+        "_is_test": True
     }),
     test = True,
     toolchains = js_binary_lib.toolchains,