MakeRequestAndExpectEventuallyConsistentResponse deferred Close() on the
Client it was passed, including a caller-supplied (injected) suite.GRPCClient
that it does not own. Closing a shared client as a side effect of one request
forces custom clients to tolerate being closed and re-used — an unusual,
undocumented contract, and a real hazard once a test re-uses the injected
client after the helper runs.

Close only the DefaultClient the helper constructs when passed nil; leave a
caller-supplied client open. Behavior is unchanged on the default path.

Signed-off-by: Aleksei Sviridkin <f@lex.la>