tests: Rewrite unit tests to not expect bridge with odp zero.

tests: Rewrite unit tests to not expect bridge with odp zero.

A future commit will make all bridges of a particular type share a
single backing datapath.  That backing datapath will have a datapath
port number of zero and bridges will be assigned other numbers.  This
commit modifies the tests so that they don't expect port zero.

It adopts the convention that bridges of type "dummy" with a name of the
form "br<n>" will be assigned a port number of 100+<n>.

Signed-off-by: Justin Pettit <jpettit@nicira.com>