Rework jspecify tests

1) Add Java use sites which are similar to Kotlin use sites;
2) Use the new test format: separated Java and Kotlin sources (Java declarations + use sites and Kotlin corresponding use sites), Java declarations + use sites will be a sharable test suite;
3) Remove any dependencies from Java sources except `Object` and jspecify annotations;
4) Introduce a new test runner which compiles Java and Kotlin sources together and dumps all descriptors.

compiler/testData/foreignAnnotationsJava8/tests/jspecify/java/testDefaults/DefaultsA.java

@@ -0,0 +1,15 @@
+import org.jspecify.annotations.*;
+
+@DefaultNotNull
+public class DefaultsA {
+    static class Test {
+        public Test foo() { return null; }
+    }
+
+    public Test defaultField = null;
+    @Nullable public Test field = null;
+
+    public Test everythingNotNullable(Test x) { return null; }
+
+    public Test explicitlyNullnessUnspecified(@NullnessUnspecified Test x) { return null; }
+}