fix: flaky integration tests

[ldez]

What does this PR do?

Waits a few milliseconds after sending SIGKILL.

Motivation

Attempts to fix flaky integration tests

FAIL: tcp_test.go:178: TCPSuite.TestCatchAllNoTLSWithHTTPS

traefik.go:76: command traefik error: error while building entryPoint tcp: error preparing server: error opening listener: listen tcp :8093: bind: address already in use

---

FAIL: headers_test.go:117: HeadersSuite.TestSecureHeadersResponses

command traefik error: error while building entryPoint web: error preparing server: error opening listener: listen tcp :8000: bind: address already in use

---

FAIL: grpc_test.go:220: GRPCSuite.TestGRPCh2cTermination

command traefik error: error while building entryPoint traefik: error preparing server: error opening listener: listen tcp :8080: bind: address already in use

---

FAIL: https_test.go:445: HTTPSSuite.TestWithClientCertificateAuthentication

traefik.go:76: command traefik error: error while building entryPoint traefik: error preparing server: error opening listener: listen tcp :8080: bind: address already in use

---

FAIL: https_test.go:692: HTTPSSuite.TestWithRootCAsContentForHTTPSOnBackend

traefik.go:76: command traefik error: error while building entryPoint traefik: error preparing server: error opening listener: listen tcp :8080: bind: address already in use

---

FAIL: https_test.go:301: HTTPSSuite.TestWithDefaultCertificate

traefik.go:76: command traefik error: error while building entryPoint websecure: error preparing server: error opening listener: listen tcp :4443: bind: address already in use

---

FAIL: acme_test.go:197: AcmeSuite.TestMultipleResolver

traefik.go:76: command traefik error: error while building entryPoint web: error preparing server: error opening listener: listen tcp :5002: bind: address already in use

More

• Added/updated tests
• Added/updated documentation

[rtribotte]

LGTM 👍

[jbdoumenjou]

LGTM

[mmatur]

LGTM