runtime: Use find_word with groups

Groups are space separated, so we can remove the 'grep -w', which has
caused problems in the past with testnames, see b3733048
("scripts: Fix the check whether testname is in the only_tests list")
and use find_word.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20211112133739.103327-3-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>