readme: add another method for running unit tests

Now that we have subprojects for all the OpenBMC based dependencies, the
tests should build and run natively using any reasonably modern Linux
distro with the correct build dependencies installed.  Add this as a
third mode for running the tests.  Add wording that indicates this is
the "preferred" mode since this is how everyone expects to run unit
tests in the first place.

Change-Id: I75e2841b224dc0e61cd3d877a8ef571167181dad
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>