Java: Rename integration test directories.

We are no longer bound to the platform-specific directories, so simplify the test organization.
If you don't want this change, just skip merging this PR. It's purely optional.

I kept the platform-specific directories around under `kotlin`,
but you could also easily merge all these together if you find them unhelpful.
I'll leave that change to you.

java/ql/integration-tests/kotlin/all-platforms/java_modifiers/test.py

@@ -0,0 +1,11 @@
+import commands
+import pathlib
+
+
+def test(codeql, java_full):
+    # Compile Java untraced. Note the Java source is hidden under `javasrc` so the Kotlin compiler
+    # will certainly reference the jar, not the source or class file for extlib.Lib
+    java_srcs = pathlib.Path("libsrc", "extlib").glob("*.java")
+    commands.run(["javac", *java_srcs, "-d", "build"])
+    commands.run("jar cf extlib.jar -C build extlib")
+    codeql.database.create(command="kotlinc test.kt -cp extlib.jar")