Add ctest for style, using clang-format

When user touches a file, that file will be fed to `git clang-format`,
and if the tool reports a new diff it will consider the test failed. The
files to check will be anything in the current workspace compared to
origin/master.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ed9fdae..9051f89 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -4,6 +4,8 @@
 
 set(TEST_WRAPPER ${CMAKE_CURRENT_BINARY_DIR}/wrapper.sh)
 
+add_test(NAME style-check COMMAND ${CMAKE_SOURCE_DIR}/scripts/style-check.sh)
+
 add_subdirectory(cc)
 add_subdirectory(python)
 add_subdirectory(lua)