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/builtins.sh

@@ -679,5 +679,12 @@ expect=$'*: times: incorrect syntax'
 actual=$(set +x; eval 'times Extra Args' 2>&1)
 [[ $actual == $expect ]] || err_exit "times with args: expected $(printf %q "$expect"), got $(printf %q "$actual"))"
 
+# ======
+# 'whence' builtin
+PATH=$tmp:$PATH $SHELL <<-\EOF || err_exit "'whence' gets wrong path on init"
+	wc=$(whence wc)
+	[[ -x $wc ]]
+EOF
+
 # ======
 exit $((Errors<125?Errors:125))