Add some simple fuzzying tests to FSFS.  These tests take a while and
must be executed manually.  Without the upcomming fixes, those tests
will crash SVN by segfaults and OOM.

The basic idea is to flip a single byte in an otherwise valid revision
file and check that we detect the corruption without crashing.  We flip
all bytes individually and use 4 different strategies for them.

In the first test, we invert all bits in the byte, the next two tests
increment / decrement it and the last test sets the value to 0.

* build.conf
  (fs-fs-fuzzy-test): Declare new build target.
  (__ALL_TESTS__): Add new target as dependency.

* subversion/tests/libsvn_fs_fs/fs-fs-fuzzy-test.c
  (): New test file implementing simple fuzzying tests.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622926 13f79535-47bb-0310-9956-ffa450edef68