Summary:
In the existing implementation of `ASSERT*`, test termination happens in `~Tester`, which is called when instance of `Tester` goes out of scope. This is the cause of many scan-build bugs.

This diff changes `ASSERT*` to terminate the test immediately. Also added one suppression in `util/signal_test.cc`

scan-build bugs
before: http://home.fburl.com/~sugak/latest/index.html
after: http://home.fburl.com/~sugak/latest2/index.html

Test Plan:
Modify some test to fail an assertion and make sure that `ASSERT*` terminated the test.

Run `make analyze` and make sure no 'Called C++ object pointer is null' and 'Dereference of null pointer' bugs reported.

Run tests and make sure no failing tests:
```lang=bash
% make check
% USE_CLANG=1 make check
```

Reviewers: meyering, lgalanis, sdong, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33381