reader: use simpler test environment constructor

A following commit wants to pass parser by exclusive reference,
which disallows passing "parser" as well as "parser.vars()"
in one function call.  This use case also doesn't make sense.
The "OperationContext::test_only_foreground" constructor is used to
inject a special "PwdEnvironment" into the context.  When we don't need
this environment, we can use a regular constructor, which already uses
"parser.vars()".

src/reader/reader.rs

@@ -7302,10 +7302,10 @@ macro_rules! validate {
                 let mut cursor_pos = in_cursor_pos;
 
                 let result = completion_apply_to_command_line(
-                    &OperationContext::test_only_foreground(
+                    &OperationContext::foreground(
                         &parser,
-                        parser.vars(),
                         Box::new(no_cancel),
+                        crate::operation_context::EXPANSION_LIMIT_DEFAULT,
                     ),
                     completion,
                     $flags,