Fix xtrace (shtests -x) for the regression tests

src/cmd/ksh93/tests/{shtests,_common}:
- When xtrace is active, set SECONDS to the float type so that
  the $SECONDS expansion in $PS4 shows fractional seconds.

src/cmd/ksh93/tests/*.sh:
- Various fixes to avoid command substitutions incorporating xtrace
  output into their results. Sometimes this is done by avoiding a
  preceding assignment on a command that redirects 2>&1 (as that
  will also redirect the preceding assignment and its xtrace,
  causing the command substitution to capture the xtrace); other
  times it was easiest to just turn off xtrace outright within the
  command substitution.

src/cmd/ksh93/tests/math.sh:
- Remove an obsolete 'fixme' note.

src/cmd/ksh93/tests/_common

@@ -30,6 +30,11 @@ alias warning='_message "$LINENO" "warning:"'
 Command=${0##*/}
 integer Errors=0
 
+if	[[ -o xtrace ]]
+then	# PS4 is set and exported in shtests
+	typeset -F SECONDS  # show fractional seconds in xtrace output
+fi
+
 if	! [[ -d $tmp && -w $tmp && $tmp == "$PWD" ]]
 then	print -r "$Command: \$tmp not set; run this from shtests. Aborting." >&2
 	exit 1