tests: fix bihash unittest error reporting

tests: fix bihash unittest error reporting

This patch fixes test_bihash_unittest in two ways:

1. The number of searches, namely tm->search_iter, defaults to 0,
thus disabling the test. This patch changes the default to 1.

2. Test errors are reported by clib_warning() instead of being
returned, thus the caller test/test_bihash.py  is never aware of them.
This patch returns the errors constructed by clib_error_return().

Type: fix

Signed-off-by: Jing Peng <[email protected]>
Change-Id: I60e99a829ebe6aa2a56e7a9332cf973afa100311