Fixed unstable test TestForwardSignals()
Fixed unstable test TestForwardSignals()

Sometimes, in the CI, it could receive a SIGURG, producing this line:

  FAIL: TestForwardSignals/signal-interrupt (0.06s)
        util_test.go:286: expected to read line "signal: interrupt" but got "signal: urgent I/O condition"

Only forward the signals we test to avoid this kind of false positive

Example of such unstable errors in CI:
https://circleci.com/gh/hashicorp/consul/153571

hashicorp/consul,hashicorp/consul,hashicorp/consul,hashicorp/consul