many new tests

many new tests

	. test70: regression test for m_out vfs race condition

The following tests use testcache.c to generate test i/o
patterns, generate random write data and verify the reads.

	. test71: blackbox full-stack test of FS operation, testing
	  using the regular VFS interface crazy i/o patterns
	  with various working set sizes, triggering only
	  primary cache, also secondary cache, and finally
	  disk i/o and verifying contents all the time
	. test72: unit test of libminixfs, implementing
	  functions it needs from -lsys and -lblockdriver
	  and the client in order to simulate a working
	  cache client and backend environment.
	. test73: blackbox test of secondary vm cache in isolation

Change-Id: I1287e9753182b8719e634917ad158e3c1e079ceb