When printing a child-process exit status in cheritest, mask it with
WEXITSTATUS().
Return EX_SOFTWARE rather than 1 from cheritest children processes on a
high-level test failure, in order to meet the expectations of the test
framework.  Low-level test failure already uses this return value.
Add a new test "test_nofault_ccall_dli_creturn" to cheritest, which sets
a register value in a handcrafted sandbox used for CCall testing; other
tests execute nothing (and a bit more nothing) in a handcrafted sandbox,
making it difficult to tell from the test whether sandboxed code actually
ran.