tests/sigchld.sh: try to fix intermittent CI fail (re: 712261c8)

Every so often, a commit's GitHub CI run throws the following
regression test failure:

    sigchld.sh[57]: expected '2 background' -- got '3' (DELAY=0.02)

When I re-run the job, the failure usually goes away.

In 712261c8 the DELAY variable was changed from 0.2 to 0.02 to
speed up the first SIGCHLD test. It's possible the GitHub CI
runners are just too slow or too heavily loaded for that.

src/cmd/ksh93/tests/sigchld.sh:
- Restore 0.2 value for 'float DELAY'.

src/cmd/ksh93/tests/sigchld.sh

@@ -20,7 +20,7 @@
 
 . "${SHTESTS_COMMON:-${0%/*}/_common}"
 
-float DELAY=${1:-0.02}
+float DELAY=${1:-0.2}
 integer FOREGROUND=10 BACKGROUND=2
 
 s=$($SHELL -c '