Change safe-buffer test structure

Change safe-buffer test structure

test_safe-buffer.cpp contains tests that ensure std::vector fails to
erase possibly confidential memory when its destructor is called, which
try to make sure the SafeBuffer testing method is valid. Since the
SafeBuffer test results may be completely wrong if these tests fail, it
would be better to merge them into one test to avoid misleading results.

I have merged the 4 tests into a single test and added some comments.

Change-Id: I9d58a7a3942a0318c0fa96047a1bdb7e708a69d4