[test] Remove extra overloads of `operator()` in tests

[SergeyKopienko]

In this PR I propose to remove extra overloads of operator() in tests:

• remove enable_if<cond, ....> before operator() in test;
• remove enable_if<!cond, ....> in the text empty operator() in test;
• check required cond inside operator() by if constexpr.

By this goal (is was proposed by @MikeDvorskiy in some of other review comments) we avoid a lot of empty code in tests.