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/attributes.sh

@@ -539,6 +539,7 @@ typeset -A expect=(
 for flag in "${!expect[@]}"
 do	unset foo
 	actual=$(
+		set +x
 		redirect 2>&1
 		export foo
 		(typeset "-$flag" foo; readonly foo; typeset -p foo)
@@ -616,6 +617,7 @@ typeset -A expect=(
 for flag in "${!expect[@]}"
 do	unset a
 	actual=$(
+		set +x
 		redirect 2>&1
 		(typeset "-$flag" a=2; typeset -p a)
 		leak=${ typeset -p a; }