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/signal.sh

@@ -454,7 +454,7 @@ echo begin
 # this extra comment disables an exec optimization
 EOF
 expect=$'^begin\n/.*/sigtest.sh: line 2: [1-9][0-9]*: Killed\n[1-9][0-9]{1,2}$'
-actual=$(LANG=C "$SHELL" -c '"$1" "$2" "$1"; echo "$?"' x "$SHELL" "$tmp/sigtest.sh" 2>&1)
+actual=$(export LANG=C; "$SHELL" -c '"$1" "$2" "$1"; echo "$?"' x "$SHELL" "$tmp/sigtest.sh" 2>&1)
 if	! [[ $actual =~ $expect ]]
 then	[[ $actual == *Killed*Killed* ]] && msg='ksh killed itself' || msg='unexpected output'
 	err_exit "$msg after child process signal (expected match to $(printf %q "$expect"); got $(printf %q "$actual"))"