Move tests/tasty/* to tests/pickling/*

These two directories were tested using the same flags, but tests/tasty
compiled all of its files at once which is usually not what is intended.

Author: smarter

compiler/test/dotc/tests.scala

@@ -355,7 +355,6 @@ class tests extends CompilerTest {
 
   @Test def tasty_dotc_util = compileDir(dotcDir, "util", testPickling)
   @Test def tasty_tools_io = compileDir(toolsDir, "io", testPickling)
-  @Test def tasty_tests = compileDir(testsDir, "tasty", testPickling)
 
   @Test def tasty_bootstrap = {
     val logging = if (false) List("-Ylog-classpath", "-verbose") else Nil