setup: exclude tests.

The inclusion of tests in the package was completely unintentional
and a result of my lack of knowledge of setuptools. If you are using
nmigen.test.utils, please copy the parts you need to your own code
because the nmigen.test code can and will change without notice.

setup.py

@@ -29,11 +29,11 @@ setup(
         "pyvcd~=0.2.0", # for nmigen.pysim
         "Jinja2~=2.11", # for nmigen.build
     ],
-    extras_require = {
+    extras_require={
         # this version requirement needs to be synchronized with the one in nmigen.back.verilog!
         "builtin-yosys": ["nmigen-yosys>=0.9.*"],
     },
-    packages=find_packages(),
+    packages=find_packages(exclude=["*.test*"]),
     entry_points={
         "console_scripts": [
             "nmigen-rpc = nmigen.rpc:main",