Introduce a helper method to quote a string in a shellsafe manner.  This
wraps the entire buffer in single quotes, escaping single-quotes and
exclamation points.

Add a function to add a search/replaced string to a git_buf.

Put the common clar test function into the tests/CMakeLists.txt for
reusability.

Other subsystems (like libgit2client) likely want to add testing.
Move clar into its own directory so that it can be reused and not
duplicated.

The test tree should - ideally - match the source tree.  The libgit2
specific tests should move into tests/libgit2.

The name of the resulting test binary is now 'libgit2_tests' for ease of
consumption of new contributors.

Add a new source directory, `util`, that contains utility functions like
buffers, vectors, etc, that that are general purpose and not necessarily
part of libgit2 itself.  These utility functions can be used by
additional projects.

We were never properly testing git_thread_exit.  Do so.

Move the settings global data teardown into its own separate function,
instead of intermingled with the global state.