[Tests] swift-package-migrate: Add test for multiple files

(cherry picked from commit f8db3a9)

Author: AnthonyLatsis

Fixtures/SwiftMigrate/InferIsolatedConformancesMigration/Sources/Fixed/Test.swift

@@ -6,9 +6,3 @@ class C: nonisolated Equatable {
     lhs.name == rhs.name
   }
 }
-
-protocol P {}
-protocol Q {}
-
-@MainActor
-struct S: nonisolated P & Q {}

Fixtures/SwiftMigrate/InferIsolatedConformancesMigration/Sources/Fixed/Test2.swift

@@ -0,0 +1,5 @@
+protocol P {}
+protocol Q {}
+
+@MainActor
+struct S: nonisolated P & Q {}

Fixtures/SwiftMigrate/InferIsolatedConformancesMigration/Sources/Test.swift

@@ -6,9 +6,3 @@ class C: Equatable {
     lhs.name == rhs.name
   }
 }
-
-protocol P {}
-protocol Q {}
-
-@MainActor
-struct S: P & Q {}

Fixtures/SwiftMigrate/InferIsolatedConformancesMigration/Sources/Test2.swift

@@ -0,0 +1,5 @@
+protocol P {}
+protocol Q {}
+
+@MainActor
+struct S: P & Q {}

Tests/CommandsTests/PackageCommandTests.swift

@@ -2112,7 +2112,7 @@ class PackageCommandTestCase: CommandsBuildProviderTestCase {
             "skipping because test environment compiler doesn't support `-print-supported-features`"
         )
 
-      func doMigration(featureName: String, expectedSummary: String) async throws {
+        func doMigration(featureName: String, expectedSummary: String) async throws {
             try await fixture(name: "SwiftMigrate/\(featureName)Migration") { fixturePath in
                 let sourcePaths: [AbsolutePath]
                 let fixedSourcePaths: [AbsolutePath]
@@ -2151,9 +2151,11 @@ class PackageCommandTestCase: CommandsBuildProviderTestCase {
             }
         }
 
+        // When updating these, make sure we keep testing both the singular and
+        // plural forms of the nouns in the summary.
         try await doMigration(featureName: "ExistentialAny", expectedSummary: "Applied 5 fix-its in 1 file")
         try await doMigration(featureName: "StrictMemorySafety", expectedSummary: "Applied 1 fix-it in 1 file")
-        try await doMigration(featureName: "InferIsolatedConformances", expectedSummary: "Applied 3 fix-its in 1 file")
+        try await doMigration(featureName: "InferIsolatedConformances", expectedSummary: "Applied 3 fix-its in 2 files")
     }
 
     func testBuildToolPlugin() async throws {