Fix remaining bug in ${var:-'{}'} (re: d087b031)

The following problems remained:

$ var=x; echo ${var:-'{}'}
x}
$ var=; echo ${var:+'{}'}
}

src/cmd/ksh93/sh/macro.c: varsub():
- Use the new ST_MOD1 state table to skip over ${var-'foo'}, etc.
  instead of ST_QUOTE. In ST_MOD1 the ' is categorised as S_LIT
  which causes the single quotes to be skipped over correctly.
  See d087b031 for more info.

src/cmd/ksh93/tests/quoting2.sh:
- Add tests for this remaining bug.
- Make the new test xtrace-proof.

Resolves: https://github.com/ksh93/ksh/issues/290 (again)

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.
 
+2021-05-03:
+
+- Completed the 2021-04-30 fix for ${var<OP>'{}'} where <OP> is '-', '+',
+  ':-' or ':+' by fixing a bug that caused an extra '}' to be output.
+
 2021-04-30:
 
 - The emacs 'ESC .' (M-.) and vi '_' commands now take shell quoting into