StdlibUnittest: run tests out of process

The test harness now can recover after test crashes, allowing:

- check for crashes themselves (without reporting them to the Python lit driver,
  which is about 10x slower -- even if CrashTracer is disabled);

- recover from unexpected test crashes and run the rest of the tests;

- this lays the groundwork for assertions that end the test execution, but
  allow the rest of the tests to run (rdar://17906801).

Note that we don't spawn a fresh process for every test.  We create a child
process and reuse it until it crashes.


Swift SVN r21090

validation-test/stdlib/StringSlicesConcurrentAppend.swift

@@ -1,5 +1,5 @@
 // RUN: %target-build-swift -parse-stdlib -Xfrontend -disable-access-control -module-name a %s -o %t.out
-// RUN: %target-run %t.out | FileCheck %s
+// RUN: %target-run %t.out
 
 import Swift
 import StdlibUnittest
@@ -109,6 +109,5 @@ StringTestCase.test("SliceConcurrentAppend") {
   barrierVar.dealloc(1)
 }
 
-StringTestCase.run()
-// CHECK: {{^}}String: All tests passed
+runAllTests()