REGTESTS: server: fix agent-check syntax and expectation

REGTESTS: server: fix agent-check syntax and expectation

Since commit 8d6c6bd ("Leak-plugging on barriers") VTest has become
stricter in its expectations, making this one fail. The agent-check
test was expecting a close on the server, which normally does not
happen before the server responds. In addition, it was really sending
"hello" (with the quotes) due to the config file syntax, which explains
why test test log reported that '"hell' was received, and complained
that 0x6f ('o') was read instead of a shutdown. This has been fixed
as well by using single-quotes.

There is no need to backport this test as it's only in 2.5.