The current echo liveness check was doing one big complex conditional
trying to incorporate both sentinel's expected ERR no such command
response and non-sentinel's actual bulk reply to ECHO.
This commit refactors the logic to check the echo response into a little
helper with different logic depending on whether or not we're connected
to a sentinel.
Additionally, we add a test to verify that we are in fact reusing
persistent connections when the user requests a persistent connection
with `RedisSentinel`.
Fixes#2148
This probably isn't a very common scenerio since we've never had someone
ask it in a decade, but it was very simple to get them working.
Primarily we just needed to test for `STDTOUT`/`STDERR` and use
`__DIR__` instead of `$_SERVER['PHP_SELF']`.
Fixes#2507
