tests: net: lib: http_server: core: Clarify existing tests

Remove pointless helper functions, the test code can simply be included
in the test case w/o code duplication.

Clarify the naming of existing test cases. Backward compatibility simply
tests HTTP1. The upgrade test wasn't really testing HTTP upgrade, so
rename the test case too. Add a new test case actually verifying HTTP
upgrade.

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