shtests: fix loose ends

src/cmd/ksh93/tests/variables.sh:
- Tolerate a bit more time for the SECONDS verification test.

src/cmd/ksh93/tests/subshell.sh:
- Replace unportable 'head -c 1' by 'dd bs=1 count=1'
- Remove unnecessary uses of 'whence'.

src/cmd/ksh93/tests/builtins.sh:
- Add a regression test for a weirdly specific 'whence' bug exposed
  by the aforementioned unneccessary uses of 'whence', which only
  shows up on my old Power Mac G5 running Mac OS X 10.3. For all I
  know it's a compiler bug, but let's add a more clear failure for
  it here, in case that happens anywhere else.

(cherry picked from commit c3898bd1e6e40874845771d33a5b37220ef0b06e)

src/cmd/ksh93/tests/subshell.sh

@@ -613,8 +613,7 @@ printf=$(whence -p printf)
 $SHELL 2> /dev/null -c '( PATH=/bin; set -o restricted) ; exit 0'  || err_exit 'restoring PATH when a subshell enables restricted exits not working'
 
 $SHELL <<- \EOF
-	wc=$(whence wc) head=$(whence head)
-	print > /dev/null  $( ( $head -c 1 /dev/zero | ( $wc -c) 3>&1 ) 3>&1) &
+	print > /dev/null  $( ( dd if=/dev/zero bs=1 count=1 2>/dev/null | ( wc -c) 3>&1 ) 3>&1) &
 	pid=$!
 	sleep .2
 	kill -9 $! 2> /dev/null && err_exit '/dev/zero in command substitution hangs'