change `-[SenTestCase invokeTest]` to create an inner autorelease pool.

This replaces the implementation of `-[SenTestCase invokeTest]` with our own implementation that creates a new autorelease pool around the invocation of setUp, <test method>, tearDown.

SenTestingKit *does* wrap test executions in autorelease pools, but it does it like this...

```
   1a. pool = [[NSAutoreleasePool alloc] init]
   2a. Announce test case started with: "Test Case '...' started."
   3a. Call setUp
   4a. Call the [test method]
   5a. Call tearDown
   6a. Announce test case ended with: "Test Case '...' failed (0.000 seconds)."
   7a. [pool drain]
```

We're modifying it to be ...

```
   1b. pool = [[NSAutoreleasePool alloc] init]
   2b. Announce test case started with: "Test Case '...' started."
>  3b. innerPool = [[NSAutoreleasePool alloc] init]
   4b. Call setUp
   5b. Call the [test method]
   6b. Call tearDown
>  7b. [innerPool drain]
   8b. Announce test case ended with: "Test Case '...' failed (0.000 seconds)."
   9b. [pool drain]
```

This gives us two benefits...

1. When an over-release bug occurs (i.e. when the autorelease pool drains and tries to free an already released object), the crash will appear to happen DURING the test rather than AFTER the test.  This is because we'll drain the pool before emitting the "Test Case '...' failed (0.000 seconds)." message.

2. It insulates otest-shim from brokenness by letting the test run completely in a different autorelease pool.  The whole situation that prompted this was a test that mock'ed `[NSDate date]` via `[OCMockObject niceMockForClass:[NSDate class]]`. OCMock works by swizzling, and it will automatically undo its swizzle when the mock object is released.

  The problem was that otest-shim's code was getting called in step [6a] above (inside of our swizzle of `+[SenTestLog testCaseDidStop:]`), and our swizzle needed to call `[NSDate date]`.  But, since we were running in the same autorelease pool as the test, the OCMockObject for NSDate hadn't yet been released and so the swizzle was still live.

  otest-shim's code was trying to call the real NSDate, but getting the swizzled version instead.

  By creating an inner pool for the test code, we can make sure the mock object + swizzle get released.