Fix 'test -t 1' in $(command substitutions)

Standard output (FD 1) tested as being on a terminal within a
command substitution, which makes no sense as the command
substitution is supposed to be catching standard output.
    ksh -c 'v=$(echo begincomsub
                [ -t 1 ] && echo oops
                echo endcomsub)
            echo "$v"'
This should not output "oops".

This is one of the many bugs with ksh93 virtual (non-forked)
subshells. On the abandoned Vashist/Rader ksh2020 branch, this bug
was fixed by changing quite a lot of code, which introduced and/or
exposed another bug:
	https://github.com/att/ast/issues/1079
	8e1e405e
	https://github.com/att/ast/issues/1088
That issue was unresolved when the ksh2020 branch was abandoned.

The safer and more conservative fix is simply forcing the subshell
to fork if we're in a non-forked command substitution and testing
'-t 1'. It is hard to imagine a situation where this would cause a
noticable performance hit.

Note that this fix does not affect ksh93-specific "shared"
non-subshell ${ command substitutions; } which are executed in the
main shell environment, so that variables survive, etcetera.
'test -t 1' continues to wrongly return true there, but command
substitutions of that form cannot be forked because that would
defeat their purpose.

src/cmd/ksh93/bltins/test.c:
- Fix 'test -t 1', '[ -t 1 ]' and '[[ -t 1 ]]' by forking the
  current subshell if it is a virtual/non-forked subshell
  (shp->subshell), and a command substitution (shp->comsub), but
  NOT a "shared" ${ command substitution; } (!shp->subshare).

src/cmd/ksh93/tests/bracket.sh:
- Add two regression tests for this issue, which were adapted from
  the Vashist/Rader ksh2020 branch.

NEWS, src/cmd/ksh93/include/version.h:
- Update.

(cherry picked from commit b8ef05e457ead65b83417699b8dd8632f855e2fa)

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-05-14"
+#define SH_RELEASE	"93u+m 2020-05-16"