TestResponderPreExpiredContext fails with go1.16 and up

[Anthony Fok]

Starting with go1.16, TestResponderPreExpiredContext fails quite often, up to 80% of the time on my computer:

--- FAIL: TestResponderPreExpiredContext (0.00s)
    st.go:41: 
        responder_test.go:118: should be == 
         	have: (<nil>) <nil>
        	want: (context.deadlineExceededError) context deadline exceeded
    st.go:41: 
        responder_test.go:119: should be == 
         	have: (bool) false
        	want: (bool) true

With go1.15, TestResponderPreExpiredContext seemingly never fails.

Tested with gock.v1 v1.1.2 and the newer commit cd5aaaa on Debian sid.