Rework TestlibTest, use JUnit console runner instead of manual suite

The problem with the manually created suite was that it was created in setUp()
and so a lot of the hard work (compilation, test case lookup) was happening in
setUp(). If any exception is thrown in setUp(), tearDown() is not called,
leaving the application (~9000+ subsequent tests) in the inconsistent state.

Support JUnit 4 tests via JUnit4TestAdapter. Previously only a small number of
test classes were actually run because this test was looking only for JUnit 3
testcases.

Delete FilesTest#relativePath because it was testing a deprecated function
and was failing if run from the project root

libraries/stdlib/test/io/Files.kt

@@ -588,16 +588,6 @@ class FilesTest {
         }
     }
 
-    @test fun relativePath() {
-        val file1 = File("src")
-        val file2 = File(file1, "kotlin")
-        val file3 = File("test")
-
-        assertEquals("kotlin", file1.relativePath(file2))
-        assertEquals("", file1.relativePath(file1))
-        assertEquals(file3.canonicalPath, file1.relativePath(file3))
-    }
-
     private fun checkFileElements(f: File, root: File?, elements: List<String>) {
         var i = 0
         assertEquals(root, f.root)