[Bazel] Make gentbl_test compatible with coverage

[GMNGeoffrey]

Summary

Add explicit coverage provider. Also remove output_to_genfiles which
isn't necessary for this test (it's just copy-pasta from gentbl_rule,
which needs it for output C++ header files).

[jpienaar]

I thought we did this one a while ago 🙂

[GMNGeoffrey]

In D106115#2884439, @jpienaar wrote:

I thought we did this one a while ago 🙂

Someone did internally. I'm backporting missing updates from the internal copy

utils/bazel/llvm-project-overlay/mlir/tblgen.bzl

def _gentbl_test_impl(ctx):
     test_args.extend(["-o", "/dev/null"])
 
     ctx.actions.write(
         ctx.outputs.executable,
         content = " ".join(test_args),
         is_executable = True,
     )
 
-    return [DefaultInfo(
+    return [
+        coverage_common.instrumented_files_info(
+            ctx,
+            source_attributes = ["td_file", "td_srcs"],
+            dependency_attributes = ["tblgen", "deps"],
+        ),
+        DefaultInfo(
             runfiles = ctx.runfiles(
                 [ctx.executable.tblgen],
                 transitive_files = trans_srcs,
             ),
-    )]
+        ),
+    ]
 
 gentbl_test = rule(
     _gentbl_test_impl,
     test = True,
     doc = "A shell test that tests the given TablegGen invocation. Note" +
           " that unlike gentbl_rule, this builds and invokes `tblgen` in the" +
           " target configuration. Takes all the same arguments as gentbl_rule" +
           " except for `out` (as it does not generate any output)",
-    # Match genrule behavior
-    output_to_genfiles = True,
     attrs = {
         "tblgen": attr.label(
             doc = "The TableGen executable run in the shell command. Note" +
                   " that this is built in the target configuration.",
             executable = True,
             cfg = "target",
         ),
         "td_file": attr.label(