Fix bionic-unit-tests in some non-standard builds

The binary bionic-unit-tests is linked with some shared libraries
and these libraries are found in ${ORIGIN}/../bionic-loader-test-libs
directory specified by runpath.  This makes it higly unlikely that
anyone would be run these files when ${ORIGIN}/../bionic-loader-test-libs
does not include proper files.

Bug: http://b/34681268
Test: run bionic-unit-tests --gtest_filter=dl*:Dl*

Change-Id: I52fc31339fdfa232c0fdafa218eadaeaf5551a8d