Add DNS resolver abstraction and mock for testable hostname resolution

[juaninf]

The test testUnknownHost was failing in some computers presumably due DNS hijacking, or other DNS mechanism. We tried to fixing this issue by replacing .com by .invalid in this-host-should-not-exist.com. The TLD .invalid is reserved by RFC 6761 and guaranteed to do not resolve.

[ic0ns]

[ERROR] Tests run: 15, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.366 s <<< FAILURE! -- in de.rub.nds.crawler.data.ScanTargetTest
[ERROR] de.rub.nds.crawler.data.ScanTargetTest.testUnknownHost -- Time elapsed: 0.013 s <<< FAILURE!
org.opentest4j.AssertionFailedError: expected: <UNRESOLVABLE> but was: <TO_BE_EXECUTED>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
	at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:182)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:177)
	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:1145)
	at de.rub.nds.crawler.data.ScanTargetTest.testUnknownHost(ScanTargetTest.java:148)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)

[juaninf]

As @ic0ns was pointing out even with .invalid there were still errors for DNS resolution. Then, we added a DNS resolver abstraction and mock for testable hostname resolution.