tests: coap_client: Refactor tests

Refactor tests to be a bit shorter, so its easier to read
and copy-paste for a new testcase

All idioms like "ret = somecall(); zasser.." are replaced with just
"zassert_ok(some_call());"

Commonly used structures are global and initialized once.

To avoid cross-test side-effects, suite_after-function is added
to cleanup all requests.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>