Delete three diagnostic tests which rendered parts of JRE

- PackageLocalClassNotImportedWithDefaultImport.kt goes to codegen multi-file
  tests. File in java.lang moved to kotlin.jvm, which seems irrelevant to the
  test, because java.lang is prohibited by the VM
- MergePackagesWithJava.kt (KT-689, Java and Kotlin in the same package) and
  kt955.kt (Unable to import from default package) are considered too obsolete
  to even test and are thus deleted

Also check that there's no more than 1000 lines in the .txt file in diagnostic
tests.

compiler/testData/codegen/boxMultiFile/packageLocalClassNotImportedWithDefaultImport/box.kt

@@ -0,0 +1,10 @@
+package a
+
+import pack.*
+
+class X : SomeClass()
+
+fun box(): String {
+    X()
+    return "OK"
+}

compiler/testData/codegen/boxMultiFile/packageLocalClassNotImportedWithDefaultImport/file1.kt

@@ -0,0 +1,3 @@
+package kotlin.jvm
+
+private class SomeClass

compiler/testData/codegen/boxMultiFile/packageLocalClassNotImportedWithDefaultImport/file2.kt

@@ -0,0 +1,3 @@
+package pack
+
+public open class SomeClass