Remove invoke_if() function in tests

[SergeyKopienko]

Remove invoke_if() function in tests: this function simple calls their argument and looks like not required:

template <typename Policy, typename F>
static void
invoke_if(Policy&&, F f)
{
    f();
}

In tests we simple replaces their calls with arguments to direct calls:

• before:

        invoke_if(exec, [&]() {
            partition(exec, iter, iter, non_const(is_even));
        });

• after:

        partition(exec, iter, iter, non_const(is_even));

[dmitriy-sobolev]

LGTM.