fix: data race in vnet.TestSSH

[nklaassen]

Fixes #55979

Flaky test detector caught a data race in VNet's TestSSH

==================
WARNING: DATA RACE
Write at 0x00c000f92870 by goroutine 393:
  github.com/gravitational/teleport/lib/vnet.(*sshAgent).setSessionKey()
      /__w/teleport.e/teleport.e/lib/vnet/ssh_agent.go:55 +0x99c
  github.com/gravitational/teleport/lib/vnet.(*sshProvider).sessionSSHConfig()
      /__w/teleport.e/teleport.e/lib/vnet/ssh_provider.go:219 +0x8be
  github.com/gravitational/teleport/lib/vnet.(*clientApplicationServiceClient).SessionSSHConfig()
      /__w/teleport.e/teleport.e/lib/vnet/client_application_service_client.go:193 +0x2f9
...

Previous read at 0x00c000f92870 by goroutine 405:
  github.com/gravitational/teleport/lib/vnet.(*sshAgent).List()
      /__w/teleport.e/teleport.e/lib/vnet/ssh_agent.go:62 +0x84
  github.com/gravitational/teleport/lib/vnet.(*forwardedAgents).forwarded()
      /__w/teleport.e/teleport.e/lib/vnet/vnet_test.go:1794 +0x168
  github.com/gravitational/teleport/lib/vnet.mustStartFakeWebProxy.func2.2()
      /__w/teleport.e/teleport.e/lib/vnet/vnet_test.go:1687 +0xd6

The changes in ssh_agent.go here fix the issue by adding a mutex around access to sshAgent.signer. The mutex isn't strictly necessary in real product code because the signer is always set before it could be read, but the test keeps a collection of sshAgents and may read them at any time, so the added mutex should resolve it.

The changes in vnet_test.go don't directly address the data race, but they were necessary for me to get TestSSH to pass at all under the race detector on my local PC. The race detector makes gVisor code extremely slow. The difference is the test no longer sets a 100ms timeout on gVisor DNS lookups and TCP dials that are required to succeed for the test to pass.