Re-fix 'test -t 1' in command substitutions (re: 090b65e7)

Since a command substitution no longer forks on non-permanently
redirecting standard output within it for a specific command,
test -t 1, [ -t 1 ], and [[ -t 1 ]] broke as follows:
v=$(test -t 1 >/dev/tty && echo ok) did not assign 'ok' to v.
This is because the assumption in tty_check() that standard output
is never on a terminal in a non-forked command substitution, added
in 55f0f8ce, was made invalid by 090b65e7.

src/cmd/ksh93/edit/edit.c: tty_check():
- Implement a new method. Return false if the file descriptor
  stream is of type SF_STRING, which is the case for non-forked
  command substitutions -- it means the sfio stream writes directly
  into a memory area. This can be checked with the sfset(3)
  function (see src/lib/libast/man/sfio.3). To avoid a segfault
  when accessing sh.sftable, we need to validate the FD first.

src/cmd/ksh93/tests/pty.sh:
- Add the above reproducer.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-05-13:
+
+- Fixed a bug with 'test -t 1' that was introduced on 2021-04-26:
+  v=$(test -t 1 >/dev/tty && echo ok) did not assign 'ok' to v.
+
 2021-05-10:
 
 - Release 1.0.0-beta.1.