[TEST] Change semantics of CHECK_TYPE directive and update testdata

Previously helpers from checkType.kt was in special package, and
  if directive was enabled then test runner (`AbstractDiagnosticTest`)
  injected additional imports to test files and removed them after test
  was completed.
It's very hard to support such behavior in new test infrastructure so
  there was a decision about changing `CHECK_TYPE`:
1. All helpers from `checkType.kt` now stays in default package
2. `CHECK_TYPE` only adds `checkType.kt` to set of analyzed files
      and don't modify their content

For test which are written in default package (most of tests actually)
  there are no changes. On the other hand if there is a test where dev
  want to use checkType functions in testfile with some package then he
  should explicitly import functions which he needed (`checkSubtype`,
  `checkType`, `_`)

compiler/testData/diagnostics/tests/j+k/kt2394.kt

@@ -4,6 +4,8 @@
 //KT-2394 java.lang.Iterable<T> should be visible as kotlin.Iterable<out T>
 package d
 
+import checkSubtype
+
 fun foo(iterable: Iterable<Int>, iterator: Iterator<Int>, comparable: Comparable<Any>) {
     checkSubtype<Iterable<Any>>(iterable)
     checkSubtype<Iterator<Any>>(iterator)

compiler/testData/diagnostics/tests/j+k/kt2641.fir.kt

@@ -5,6 +5,8 @@ package a
 import java.util.Iterator
 import java.lang.Comparable as Comp
 
+import checkSubtype
+
 fun bar(any: Any): java.lang.Iterable<Int>? {
     val a: java.lang.Comparable<String>? = null
     val b: Iterable<Integer>
@@ -15,4 +17,4 @@ fun bar(any: Any): java.lang.Iterable<Int>? {
     }
     any as Iterator<*>
     return null
-}
+}

compiler/testData/diagnostics/tests/j+k/kt2641.kt

@@ -5,6 +5,8 @@ package a
 import <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.util.Iterator<!>
 import <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.lang.Comparable<!> as Comp
 
+import checkSubtype
+
 fun bar(any: Any): <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.lang.Iterable<Int><!>? {
     val <!UNUSED_VARIABLE!>a<!>: <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.lang.Comparable<String><!>? = null
     val <!UNUSED_VARIABLE!>b<!>: Iterable<<!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>Integer<!>>
@@ -15,4 +17,4 @@ fun bar(any: Any): <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.lang.Iterable<Int><!>
     }
     any as <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>Iterator<*><!>
     return null
-}
+}