Apply clang-format universally.

* It is tough to contribute to a project that doesn't use a formatter,
* It is extra hard to contribute to a project which has configured the formatter, but ignores it for some files
* Code formatting makes it harder to hide obscure / weird bugs by accident or on purpose,

Let's rip the bandaid off?

Note that PRs currently in flight should be able to be merged relatively easily by applying `clang-format` to their tip prior to merge.

src/libutil-test-support/string_callback.cc

@@ -8,4 +8,4 @@ void observe_string_cb(const char * start, unsigned int n, void * user_data)
     *user_data_casted = std::string(start);
 }
 
-}
+} // namespace nix::testing