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/shtests

@@ -215,6 +215,7 @@ shift $OPTIND-1
 
 if	(( debug )) || [[ $trace ]]
 then	export PS4='+ [${SECONDS:+${SECONDS%????}s|}${.sh.pid:+P${.sh.pid},}${.sh.subshell:+S${.sh.subshell},}${.sh.file:+${.sh.file#${.sh.file%/*/*}/},}${.sh.fun:+${.sh.fun},}${LINENO:+L$LINENO,}e$?] '
+	typeset -F SECONDS  # show fractional seconds in xtrace output
 	if	(( debug ))
 	then	set -x
 	fi