unified: Also test for string with interpolation

This worked, but it's nice to have a test for it

unified/extractor/tests/corpus/swift/control-flow/switch-expression-pattern.output

@@ -1,6 +1,6 @@
 // Arbitrary expressions may appear in pattern context
 switch x {
-case y, y + 1, -y, y...z, "foo", 4, foo().bar, .baz, [42], ["a": 1], try y, y!, y is T, await y:
+case y, y + 1, -y, y...z, "foo", 4, foo().bar, .baz, [42], ["a": 1], try y, y!, y is T, await y, "interpolate \(y)":
     print("expr")
 case true ? y : z:
     print("ternary is also valid")
@@ -189,6 +189,7 @@ sourceFile
                                       name: identifier "T"
                                   isKeyword: is
                         switchCaseItem
+                          trailingComma: ,
                           pattern:
                             expressionPattern
                               expression:
@@ -197,6 +198,27 @@ sourceFile
                                     declReferenceExpr
                                       baseName: identifier "y"
                                   awaitKeyword: await
+                        switchCaseItem
+                          pattern:
+                            expressionPattern
+                              expression:
+                                stringLiteralExpr
+                                  closingQuote: "
+                                  openingQuote: "
+                                  segments:
+                                    stringSegment
+                                      content: stringSegment "interpolate "
+                                    expressionSegment
+                                      leftParen: (
+                                      rightParen: )
+                                      backslash: \
+                                      expressions:
+                                        labeledExpr
+                                          expression:
+                                            declReferenceExpr
+                                              baseName: identifier "y"
+                                    stringSegment
+                                      content: stringSegment
                   statements:
                     codeBlockItem
                       item:
@@ -373,6 +395,19 @@ top_level
                             name_expr
                               identifier: identifier "y"
                           operator: prefix_operator "await"
+                    expr_equality_pattern
+                      expr:
+                        string_interpolation_expr
+                          element:
+                            string_literal "interpolate "
+                            call_expr
+                              callee: builtin_expr "interpolation"
+                              argument:
+                                argument
+                                  value:
+                                    name_expr
+                                      identifier: identifier "y"
+                            string_literal
               body:
                 block
                   stmt:

unified/extractor/tests/corpus/swift/control-flow/switch-expression-pattern.swift

@@ -1,6 +1,6 @@
 // Arbitrary expressions may appear in pattern context
 switch x {
-case y, y + 1, -y, y...z, "foo", 4, foo().bar, .baz, [42], ["a": 1], try y, y!, y is T, await y:
+case y, y + 1, -y, y...z, "foo", 4, foo().bar, .baz, [42], ["a": 1], try y, y!, y is T, await y, "interpolate \(y)":
     print("expr")
 case true ? y : z:
     print("ternary is also valid")