oeqa/selftest/glibc: Handle incorrect encoding issuesin glibc test results

We dont control the test output and sometimes see badly encoded characters.
These don't affect the operation of the code we're running. Use
errors='replace' to avoid those issues and the resulting test failures.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3939b192)
Signed-off-by: Steve Sakoman <steve@sakoman.com>