Fix app-tap/logger_pipe test

'echo $TEST_VAR' exits almost instantly, which may be detected by
log_pipe_init(), resulting in the test failure:

    app-tap/logger_pipe.test.lua                        [ fail ]

    Test failed! Result content mismatch:
    --- app-tap/logger_pipe.result  Wed Jan 17 14:15:24 2018
    +++ app-tap/logger_pipe.reject  Wed Jan 17 14:47:35 2018
    @@ -1 +1,3 @@
     48
     +IllegalParams: logger process died
     +failed to initialize logging subsystem

Fix this by appending 'cat > /dev/null' to the pipe logger command.

Fixes 0ab233cd Don't discard environment variables in pipe logger

Closes #3048

test/app-tap/logger_pipe.test.lua

 #!/usr/bin/env tarantool
 
 os.setenv('TEST_VAR', '48')
-box.cfg { log = '|echo $TEST_VAR' }
+box.cfg { log = '|echo $TEST_VAR; cat > /dev/null' }
 os.exit(0)