tests: net: tcp: Replace test case numbers with enums

So far test cases in TCP test suite were represented by magic numbers.
With the increasing size of this test suite, it started to become
troublesome to follow, which number represents which test case or what
number can a new test case get.

Introduce a new enum for the test suite, which can be used to represent
individual test case. It simplifies adding new test cases (test case ids
are now defined in a single place, so easier to extend), and also
improves code readability.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>