libxml2: backport -timeout flag for testlimits

libxml2: backport -timeout flag for testlimits

[YOCTO #15912]

A commit was recently merged upstream to make the testlimits test's
timeout configurable. This test fails intermittently on the AB under
heavy load (particularly on qemuriscv64), so backport the change and
adjust the timeout from two seconds to five.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>