test: fix flaky app/fiber_channel test

The test has three known failures:

 1. Happens, because the test doesn't wait for a cancelled fiber
    to exit. As a result, it keeps reading from the test channel.
    Fix this by waiting for the test fiber to be dead for sure.
    ```
    --- app/fiber_channel.result    Sun Apr 26 17:45:32 2020
    +++ var/050_app/fiber_channel.result    Tue May 12 05:10:14 2020
    @@ -200,381 +200,14 @@
     ...
     ch:count()
     ---
    -- 1
    +- 0
     ...
     ch:is_full()
     ---
    -- true
    +- false
     ...
     ch:is_empty()
     ---
    -- false
    +- true
     ...
     ch:get(box.info.pid) == box.info.pid
    ```
 2. The test writes 16 values to a table, but waits for only 15 values.
    Fix this and also cancel test fibers before proceeding to the next
    test case.
    ```
    --- app/fiber_channel.result	Fri Feb 21 11:21:32 2020
    +++ var/049_app/fiber_channel.result	Tue May 12 01:05:37 2020
    @@ -346,7 +346,6 @@
       - 42
       - 43
       - 44
    -  - 45
     ...
     ch = fiber.channel(1)
     ---
    ```
 3. The test uses a 100 iteration loop to wait for a condition to
    become true. If the host is busy, 100 may be not enough. Let's
    use test_run:wait_cond() instead - it's more reliable.
    ```
    --- app/fiber_channel.result	Fri Feb 21 12:08:07 2020
    +++ app/fiber_channel.reject	Fri May  8 08:20:52 2020
    @@ -487,7 +487,7 @@
     ...
     count > 2000, #test_res, test_res;
     ---
    -- true
    +- false
     - 10
    unit/fiber_channel.test                                         [ pass ]
     - - true
       - true
    ```

Closes #4961