Support tests that use OHHTTPStubs in Swift Package Manager

[azarovalex]

Fixes #494.

Looks like OHHTTPStubs don't have Linux support because OHHTTPStubs uses objc/runtime to swizzle URLSesson methods here.

OHHTTPStubs on macOS through SMP works fine, there is a minor problem with module names – when we import OHHTTPStubs through Carthage, the name of the module is OHHTTPStubs, and when through SMP, the name is OHHTTPStubsSwift. Even though the content of the modules is the same. Currently, I'm solving this issue with a conditional compilation like this:

import OHHTTPStubs
#if SWIFT_PACKAGE
import OHHTTPStubsSwift
#endif

In theory, we can create our own empty package that will use @_exported statements and then include this everywhere, but this API is private.

Also, we need to replace every bundle creating with Bundle.module.
If you have any suggestions on how to run these tests on Linux or how to simplify import statements, feel free to add a comment.

[1ec5]

The changes look pretty reasonable to me, including the conditional compilation. The Bundle.module changes are fine because, elsewhere in the test target, we declare that constant conditionally for Carthage. I just have one comment about a more straightforward way to declare the conditional OHHTTPStubs dependency, but it isn’t critical.