test/[: use a shell state bit (re: 7003aba4)

Instead of a global 'sh_in_test_builtin' integer flag, it is nicer
to use the mechanism for shell state bits, which was designed for
this sort of thing.

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/defs.c:
- Remove global sh_in_test_builtin integer.
- Define new SH_INTESTCMD state bit.

src/cmd/ksh93/bltins/test.c: _ERROR_exit_b_test(), b_test():
- Use the new state bit.

src/cmd/ksh93/sh/defs.c

@@ -46,4 +46,3 @@ char *sh_lexstates[ST_NONE] = {0};
 struct jobs	job = {0};
 int32_t		sh_mailchk = 600;
 
-int		sh_in_test_builtin = 0;