Tests randomly fail since 1.0.16

[Robert-André Mauchin]

Since 1.0.16 (tested git tip too), the tests are failing randomly with:

--- FAIL: TestMatchMock (0.00s)
    st.go:41: 
        matcher_test.go:154: should be == 
         	have: (*gock.Mocker) &{disabled:true mutex:{state:0 sema:0} matcher:0xc0000aa120 request:0xc0004124e0 response:0xc0004be6e0}
        	want: (*gock.Mocker) &{disabled:false mutex:{state:0 sema:0} matcher:0xc00043c408 request:0xc0004bdba0 response:0xc0000c7ea0}
FAIL
exit status 1
FAIL	gopkg.in/h2non/gock.v1	0.015s

Tested on Fedora Rawhide and 33, with respectively Go 1.16 beta 1 on Go 1.15.6