Fix header_generator_test (#88)

* Fix header_generator_test

Due to a missing `unittest.main()` call, the header_generator_test was
not actually running any tests when run with bazel/run directly.
However, the failure was noticed by `python -m unittest discover`. Fixed
the header_generator_test in the following ways:

 - Import test_util (tests failed due to missing import).
 - Change attribute_checker.normalize_and_verify to
   header_generator.generate_header.
 - Remove unused imports.
 - Make appropriate BUILD changes.
 - Add unittest.main() call so that unit tests are run.

Additionally, this change adds a (blank) __init__.py to compiler/ and
compiler/util/ so that they are importable by the unittest module. This
means that running `python -m unittest discover -p '*_test.py'` from the
main emboss repository should run all python tests in the compiler - and
could be a good sanity check to make sure all python tests pass, even
when `bazel test compiler/...` could miss tests (not declared in BUILD
or missing the unittest.main() call).

* Move front_end/test_util to util/test_util
diff --git a/compiler/front_end/constraints_test.py b/compiler/front_end/constraints_test.py
index 9bf5a8e..b214107 100644
--- a/compiler/front_end/constraints_test.py
+++ b/compiler/front_end/constraints_test.py
@@ -18,9 +18,9 @@
 from compiler.front_end import attributes
 from compiler.front_end import constraints
 from compiler.front_end import glue
-from compiler.front_end import test_util
 from compiler.util import error
 from compiler.util import ir_util
+from compiler.util import test_util
 
 
 def _make_ir_from_emb(emb_text, name="m.emb"):