[Test] CoroutineAccessors: Added with all four.

Author: nate-chandler

Tests/SwiftParserTest/DeclarationTests.swift

@@ -3349,6 +3349,25 @@ final class DeclarationTests: ParserTestCase {
       """,
       experimentalFeatures: .coroutineAccessors
     )
+    assertParse(
+      """
+      public var i: Int {
+        _read {
+          yield _i
+        }
+        read {
+          yield _i
+        }
+        _modify {
+          yield &_i
+        }
+        modify {
+          yield &_i
+        }
+      }
+      """,
+      experimentalFeatures: .coroutineAccessors
+    )
     assertParse(
       """
       var i_rm: Int {