x/net/quic: TestStreamsCreateConcurrency failures

[gopherbot]

#!watchflakes
default <- pkg == "golang.org/x/net/quic" && test == "TestStreamsCreateConcurrency"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestStreamsCreateConcurrency
panic: test timed out after 1h40m0s
	running tests:
		TestStreamsCreateConcurrency (1h39m59s)

goroutine 842 [running]:
testing.(*M).startAlarm.func1()
	/home/swarming/.swarming/w/ir/x/w/goroot/src/testing/testing.go:2795 +0x605
created by time.goFunc
	/home/swarming/.swarming/w/ir/x/w/goroot/src/time/sleep.go:215 +0x45
...
net.(*UDPConn).readMsg(0xc0001c4730, {0xc000a91200, 0x5c0, 0x5c0}, {0xc00051b900, 0x78, 0x78})
	/home/swarming/.swarming/w/ir/x/w/goroot/src/net/udpsock_posix.go:101 +0x290
net.(*UDPConn).ReadMsgUDPAddrPort(0xc0001c4730, {0xc000a91200, 0x5c0, 0x5c0}, {0xc00051b900, 0x78, 0x78})
	/home/swarming/.swarming/w/ir/x/w/goroot/src/net/udpsock.go:200 +0xbb
golang.org/x/net/quic.(*netUDPConn).Read(0xc000a071a0, 0xc000191e10)
	/home/swarming/.swarming/w/ir/x/w/targetrepo2821269253/quic/udp_msg.go:85 +0x1d9
golang.org/x/net/quic.(*Endpoint).listen(0xc0007494a0)
	/home/swarming/.swarming/w/ir/x/w/targetrepo2821269253/quic/endpoint.go:234 +0x116
created by golang.org/x/net/quic.newEndpoint in goroutine 868
	/home/swarming/.swarming/w/ir/x/w/targetrepo2821269253/quic/endpoint.go:112 +0x58b

— watchflakes