du: tests: make H_flag tests more strict about output requirements

The current version of this test will effectively pass as long as one of the
specified paths is in the output, and it could even be a subset of one of
the paths.

Strengthen up the test a little bit:

• Specify beginning/end anchors for each path
• Add egrep -v checks to make sure we don't have any *additional* paths
• Ratchet down paths2 to exactly the two paths we expect to appear

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D27984