Basically the network wasn't responding and detection failed, but also crashed in so doing.
Stack trace follows, but doesn't mention networktest code. The crash happened in the runtime.
failed MSpanList_Insert 0x2ff000 0xd3a61c1e713 0x0 0x0
fatal error: MSpanList_Insert
runtime stack:
runtime.throw(0x1e1030, 0x10)
/usr/lib/go/src/runtime/panic.go:530 +0x90 fp=0x7fff5fbff600 sp=0x7fff5fbff5e8
runtime.(*mSpanList).insert(0x2ade08, 0x2ff000)
/usr/lib/go/src/runtime/mheap.go:933 +0x293 fp=0x7fff5fbff630 sp=0x7fff5fbff600
runtime.(*mheap).freeSpanLocked(0x2ad600, 0x2ff000, 0x100, 0x0)
/usr/lib/go/src/runtime/mheap.go:809 +0x4be fp=0x7fff5fbff698 sp=0x7fff5fbff630
runtime.(*mheap).grow(0x2ad600, 0x8, 0x0)
/usr/lib/go/src/runtime/mheap.go:675 +0x2a0 fp=0x7fff5fbff6f0 sp=0x7fff5fbff698
runtime.(*mheap).allocSpanLocked(0x2ad600, 0x1, 0x0)
/usr/lib/go/src/runtime/mheap.go:553 +0x4e3 fp=0x7fff5fbff748 sp=0x7fff5fbff6f0
runtime.(*mheap).alloc_m(0x2ad600, 0x1, 0x15, 0x0)
/usr/lib/go/src/runtime/mheap.go:437 +0x119 fp=0x7fff5fbff778 sp=0x7fff5fbff748
runtime.(*mheap).alloc.func1()
/usr/lib/go/src/runtime/mheap.go:502 +0x41 fp=0x7fff5fbff7a8 sp=0x7fff5fbff778
runtime.systemstack(0x7fff5fbff7c8)
/usr/lib/go/src/runtime/asm_amd64.s:307 +0xab fp=0x7fff5fbff7b0 sp=0x7fff5fbff7a8
runtime.(*mheap).alloc(0x2ad600, 0x1, 0x10000000015, 0x2dbef)
/usr/lib/go/src/runtime/mheap.go:503 +0x63 fp=0x7fff5fbff7f8 sp=0x7fff5fbff7b0
runtime.(*mcentral).grow(0x2af200, 0x0)
/usr/lib/go/src/runtime/mcentral.go:209 +0x93 fp=0x7fff5fbff860 sp=0x7fff5fbff7f8
runtime.(*mcentral).cacheSpan(0x2af200, 0x2aa1c8)
/usr/lib/go/src/runtime/mcentral.go:89 +0x47d fp=0x7fff5fbff8a0 sp=0x7fff5fbff860
runtime.(*mcache).refill(0x2fb000, 0x15, 0x7fff5fbff908)
/usr/lib/go/src/runtime/mcache.go:119 +0xcc fp=0x7fff5fbff8d8 sp=0x7fff5fbff8a0
runtime.mallocgc.func2()
/usr/lib/go/src/runtime/malloc.go:642 +0x2b fp=0x7fff5fbff8f8 sp=0x7fff5fbff8d8
runtime.systemstack(0x7fff5fbff998)
/usr/lib/go/src/runtime/asm_amd64.s:307 +0xab fp=0x7fff5fbff900 sp=0x7fff5fbff8f8
runtime.mallocgc(0x180, 0x1bb220, 0x0, 0x800000000)
/usr/lib/go/src/runtime/malloc.go:643 +0x869 fp=0x7fff5fbff9d8 sp=0x7fff5fbff900
runtime.newobject(0x1bb220, 0x2aa4d0)
/usr/lib/go/src/runtime/malloc.go:781 +0x42 fp=0x7fff5fbffa00 sp=0x7fff5fbff9d8
runtime.malg(0x8000, 0x2aa740)
/usr/lib/go/src/runtime/proc.go:2634 +0x27 fp=0x7fff5fbffa38 sp=0x7fff5fbffa00
runtime.mpreinit(0x2aaa80)
/usr/lib/go/src/runtime/os1_darwin.go:140 +0x1f fp=0x7fff5fbffa50 sp=0x7fff5fbffa38
runtime.mcommoninit(0x2aaa80)
/usr/lib/go/src/runtime/proc.go:494 +0x105 fp=0x7fff5fbffa98 sp=0x7fff5fbffa50
runtime.schedinit()
/usr/lib/go/src/runtime/proc.go:434 +0x79 fp=0x7fff5fbffae0 sp=0x7fff5fbffa98
runtime.rt0_go(0x7fff5fbffb18, 0x1, 0x7fff5fbffb18, 0x0, 0x0, 0x1, 0x7fff5fbffc50, 0x0, 0x7fff5fbffc5a, 0x7fff5fbffc76, ...)
/usr/lib/go/src/runtime/asm_amd64.s:138 +0x132 fp=0x7fff5fbffae8 sp=0x7fff5fbffae0
Basically the network wasn't responding and detection failed, but also crashed in so doing.
Stack trace follows, but doesn't mention networktest code. The crash happened in the runtime.