tests: enable/disable internet-dependent tests at runtime

[tchaikov]

Previously, some tests in dns_test.cc and tls_test.cc that required internet connectivity were conditionally compiled using #if SEASTAR_TESTING_WITH_NETWORKING. However, several other internet-dependent tests were not properly disabled,
causing test failures in environments without internet access.

This change:

• Replaces compile-time guards with runtime conditional execution using boost::unit_test::enable_if<SEASTAR_TESTING_WITH_NETWORKING> for tests requiring internet
• Uses boost::unit_test::enable_if<!SEASTAR_TESTING_WITH_NETWORKING> for alternative tests that provide equivalent coverage without internet access

This approach improves maintainability, enables running the test suite in offline environments, and makes internet dependencies more explicit in the test code.

[tchaikov]

cc @Comrade88

[Comrade88]

now you reapeat these typedef's in every test file. may be it is better to put them somewhere central or even introduce special SEASTAR_TEST_CASE macro wrappers?

[tchaikov]

now you reapeat these typedef's in every test file. may be it is better to put them somewhere central or even introduce special SEASTAR_TEST_CASE macro wrappers?

so far, we have two test files including this decorator. once it repeats itself in more places, probably we should indeed have a central place for it. what do you think?

[Comrade88]

at the moment before my pr there was only one place in the project with a conditional disabling of online tests. I encountered a problem, found this place, understood what needed to be changed, and decided that this is how such issues are solved here.
later someone else will come, see the proposed typedefs and copy them to the next file, because this is how it is done here. in my opinion, it is better to immediately provide prospective users with a good example.