resolv: Serialize processing in resolv/tst-resolv-txnid-collision

When switching name servers, response processing by two server
threads clobbers the global test state.  (There is still some
risk that this test is negatively impact by packet drops and
packet reordering, but this applies to many of the resolver tests
and is difficult to avoid.)

Fixes commit f1f00c07 ("resolv:
Handle transaction ID collisions in parallel queries (bug 26600)").