Follow-up to r1590405: Add missing test sources

* subversion/tests/libsvn_subr/bit-array-test.c
  (): Some unit tests for svn_bit_array__*

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590430 13f79535-47bb-0310-9956-ffa450edef68