Add additional significance searcher tests

[MariusArhaug]

[bjorncs]

PR build broke:

13:06:19 [ERROR] com.yahoo.search.significance.test.SignificanceSearcherTest.testSignificanceValueOnRecursiveQuery -- Time elapsed: 0.008 s <<< FAILURE!
13:06:19 org.opentest4j.AssertionFailedError: expected: <1.4816045409242156> but was: <1.916922612182061>
13:06:19 	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
13:06:19 	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
13:06:19 	at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
13:06:19 	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:182)
13:06:19 	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:177)
13:06:19 	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:1145)
13:06:19 	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
13:06:19 	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
13:06:19 	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
13:06:19 	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
13:06:19 	at com.yahoo.test.JunitCompat.invokeAssert(JunitCompat.java:56)
13:06:19 	at com.yahoo.test.JunitCompat.invokeAssert(JunitCompat.java:41)
13:06:19 	at com.yahoo.test.JunitCompat.assertEquals(JunitCompat.java:19)
13:06:19 	at com.yahoo.search.significance.test.SignificanceSearcherTest.testSignificanceValueOnRecursiveQuery(SignificanceSearcherTest.java:156)
13:06:19 	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
13:06:19 	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
13:06:19 	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)