Use HttpWaitStrategy for SimpleNginxTest

[rnorth]

To ensure proper waits and avoid race conditions (flaky test)

Since the Gradle migration we've been seeing a lot of the following stack trace. I have no idea why, but I suspect an improvement in test execution speed might have uncovered a previously hidden timing bug in our tests:

Gradle Test Executor 7 > org.testcontainers.junit.SimpleNginxTest > testSimple FAILED
   java.net.SocketException: Unexpected end of file from server
       at sun.net.www.http.HttpClient.parseHTTPHeader(HttpClient.java:851)
       at sun.net.www.http.HttpClient.parseHTTP(HttpClient.java:678)
       at sun.net.www.http.HttpClient.parseHTTPHeader(HttpClient.java:848)
       at sun.net.www.http.HttpClient.parseHTTP(HttpClient.java:678)
       at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1587)
       at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1492)
       at org.testcontainers.junit.SimpleNginxTest.testSimple(SimpleNginxTest.java:49)

This PR simply attaches an HttpWaitStrategy to the test. We could have added this to the NginxContainer itself, but I felt that would be counterproductive: users will typically need to customise the http wait strategy according to their nginx configuration, so we should leave it to them.

[kiview]

@rnorth I feel like having a HttpWaitStrategy as the default would be helpful (I think many first time users of TC don't even know about wait strategies in the beginning). Or what are the special use cases where users would need to customize the Strategy? Shouldn't status code 200 suffice?