Remove unused CheckerTest base type (#1035)

Author: d-ronnqvist

Tests/SwiftDocCTests/Checker/Checkers/InvalidAdditionalTitleTests.swift

@@ -12,7 +12,7 @@ import XCTest
 @testable import SwiftDocC
 import Markdown
 
-class InvalidAdditionalTitleTests: XCTestCase, CheckerTest {
+class InvalidAdditionalTitleTests: XCTestCase {
     func gatherProblems(for document: Document) -> [Problem] {
         var checker = InvalidAdditionalTitle(sourceFile: nil)
         checker.visit(document)

Tests/SwiftDocCTests/Checker/Checkers/MissingAbstractTests.swift

@@ -12,7 +12,7 @@ import XCTest
 @testable import SwiftDocC
 import Markdown
 
-class MissingAbstractTests: XCTestCase, CheckerTest {
+class MissingAbstractTests: XCTestCase {
 
     func gatherProblems(for document: Document) -> [Problem] {
         var checker = MissingAbstract(sourceFile: nil)