Test that MTU probing is only enabled on platforms that guarantee the DF bit is set

[camshaft]

Problem:

In #1737, I added the check to lower the max_mtu if the syscall to set the DF bit failed. We should add a test on platforms where this is true to preserve this logic.

Solution:

Add a test for windows and macOS that checks that, even though the application configured a large max_mtu the emitted event says that it's 1200 bytes.

[camshaft]

Apparently macOS and iOS 11 added support for this so this should include probing for this feature.