healthcheck correct connect result

Based on user reports, a `connect` test can observer a non-"Can't
connect" error message.

Because this passes other tests, like innodb_initialized might
succeeded.

The final test -z "$connect_s" is also true, leaving the user
with an incorrect test result.

Maybe the healthcheck cnf hasn't been created yet?

Either way, ruggardise the final test to ensure a healthy connect
test occured.

Narrow down the scope of healthcheck --connect failures
to the mariadb client errors "Can't connect".

May as well look at same @skip-networking system variable
on TCP just in case.

28000 errors, per
https://mariadb.com/kb/en/mariadb-error-code-reference/ are client
errors after a connection, indicating that the start of a tcp
connection occured.

Closes #610