Cleanup files in tests after R CMD build

The .o files in the src/lib directory of exampleC package are
non-standard and are not automatically cleaned up by `R CMD BUILD`. So
they need to be cleaned manually.

exampleC/src/lib/*.o also contains the compile dir which is a temp
directory and makes our reproducibility test fail because exampleC.so is
not reproducible.

tests/exampleC/cleanup

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Copyright 2020 The Bazel Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Remove any non-standard files generated by `R CMD BUILD` and not
+# automatically cleaned up.
+# src/lib/*.o files also contain the compile dir which is a temp dir and makes
+# them not byte reproducible.
+rm -f src/lib/*.o