Mark all test classes as final.

From the comment in the review on
#1480, doing this for all tests.

Author: thomasvl

Tests/SwiftProtobufPluginLibraryTests/Test_Descriptor.swift

@@ -12,7 +12,7 @@ import XCTest
 import SwiftProtobuf
 import SwiftProtobufPluginLibrary
 
-class Test_Descriptor: XCTestCase {
+final class Test_Descriptor: XCTestCase {
 
   func testParsing() throws {
     let fileSet = try Google_Protobuf_FileDescriptorSet(serializedData: fileDescriptorSetData)

Tests/SwiftProtobufPluginLibraryTests/Test_NamingUtils.swift

@@ -12,7 +12,7 @@ import XCTest
 import SwiftProtobuf
 @testable import SwiftProtobufPluginLibrary
 
-class Test_NamingUtils: XCTestCase {
+final class Test_NamingUtils: XCTestCase {
 
   func testTypePrefix() throws {
     // package, swiftPrefix, expected

Tests/SwiftProtobufPluginLibraryTests/Test_ProtoFileToModuleMappings.swift

@@ -30,7 +30,7 @@ extension ProtoFileToModuleMappings.LoadError: Equatable {
 
 fileprivate typealias FileDescriptorProto = Google_Protobuf_FileDescriptorProto
 
-class Test_ProtoFileToModuleMappings: XCTestCase {
+final class Test_ProtoFileToModuleMappings: XCTestCase {
 
   func test_Initialization() {
     // ProtoFileToModuleMappings always includes mappings for the protos that

Tests/SwiftProtobufPluginLibraryTests/Test_SwiftLanguage.swift

@@ -15,7 +15,7 @@
 import XCTest
 import SwiftProtobufPluginLibrary
 
-class Test_SwiftLanguage: XCTestCase {
+final class Test_SwiftLanguage: XCTestCase {
     func testIsValidSwiftIdentifier() {
         let cases = [
             "H9000",

Tests/SwiftProtobufPluginLibraryTests/Test_SwiftProtobufNamer.swift

@@ -13,7 +13,7 @@ import SwiftProtobuf
 import SwiftProtobufPluginLibrary
 import SwiftProtobufTestHelpers
 
-class Test_SwiftProtobufNamer: XCTestCase {
+final class Test_SwiftProtobufNamer: XCTestCase {
 
   func testEnumValueHandling_AliasNameMatches() throws {
     let txt = [

Tests/SwiftProtobufTests/Test_AllTypes.swift

@@ -19,7 +19,7 @@ import Foundation
 import SwiftProtobuf
 import XCTest
 
-class Test_AllTypes: XCTestCase, PBTestHelpers {
+final class Test_AllTypes: XCTestCase, PBTestHelpers {
     typealias MessageTestType = SwiftProtoTesting_TestAllTypes
 
     // Custom decodeSucceeds that also does a round-trip through the Empty

Tests/SwiftProtobufTests/Test_AllTypes_Proto3.swift

@@ -18,7 +18,7 @@
 import Foundation
 import XCTest
 
-class Test_AllTypes_Proto3: XCTestCase, PBTestHelpers {
+final class Test_AllTypes_Proto3: XCTestCase, PBTestHelpers {
     typealias MessageTestType = SwiftProtoTesting_Proto3_TestAllTypes
 
     // Custom decodeSucceeds that also does a round-trip through the Empty

Tests/SwiftProtobufTests/Test_AllTypes_Proto3_Optional.swift

@@ -18,7 +18,7 @@
 import Foundation
 import XCTest
 
-class Test_AllTypes_Proto3_Optional: XCTestCase, PBTestHelpers {
+final class Test_AllTypes_Proto3_Optional: XCTestCase, PBTestHelpers {
     typealias MessageTestType = SwiftProtoTesting_TestProto3Optional
 
     // Custom decodeSucceeds that also does a round-trip through the Empty

Tests/SwiftProtobufTests/Test_Any.swift

@@ -19,7 +19,7 @@ import Foundation
 import XCTest
 import SwiftProtobuf
 
-class Test_Any: XCTestCase {
+final class Test_Any: XCTestCase {
 
     func test_Any() throws {
         var content = SwiftProtoTesting_TestAllTypes()

Tests/SwiftProtobufTests/Test_Api.swift

@@ -18,7 +18,7 @@ import Foundation
 import XCTest
 import SwiftProtobuf
 
-class Test_Api: XCTestCase, PBTestHelpers {
+final class Test_Api: XCTestCase, PBTestHelpers {
     typealias MessageTestType = Google_Protobuf_Api
 
     func testExists() {