review erfcx(Inf) and cerfcx(Inf)

In realtest.c, this line is currently disabled:

// iTest(&result, "erfcx", cerfcx, erfcx);

Otherwise we get a failure under Debian:

5/6 Test #3: realtest .........................***Failed    0.08 sec
failure in subtest 120010: erfcx(Inf)
- fct value                    0
- expected                  -nan
=> error    inf above limit  1e-13
1/142515 tests failed