Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.Approved by: re (implicit)Sponsored by: The FreeBSD Foundation
MFC r245383,245392,247190,249220,251180,251797: New sh testcases.These already work on stable/9.
sh: Add some testcases related to subshells.These failed in earlier attempts to execute more subshells without forking.The patches are uncommitted.