Revert "mask out tests that are failing on Linux (#996)" (#1001)

This reverts commit 3d58514.

Author: anferbui

Tests/SwiftDocCTests/LinkTargets/LinkDestinationSummaryTests.swift

@@ -436,9 +436,7 @@ class ExternalLinkableTests: XCTestCase {
             XCTAssertEqual(decoded, summary)
         }
     }
-
-    // TODO: Remove this #if once https://github.com/apple/swift-corelibs-foundation/issues/5028 is resolved
-    #if os(macOS)
+    
     func testVariantSummaries() throws {
         let (bundle, context) = try testBundleAndContext(named: "MixedLanguageFramework")
         let converter = DocumentationNodeConverter(bundle: bundle, context: context)
@@ -593,8 +591,7 @@ class ExternalLinkableTests: XCTestCase {
             XCTAssertEqual(decoded, summary)
         }
     }
-    #endif
-
+    
     func testDecodingLegacyData() throws {
         let legacyData = """
         {

Tests/SwiftDocCUtilitiesTests/ConvertActionTests.swift

@@ -17,8 +17,7 @@ import Markdown
 @testable import SwiftDocCTestUtilities
 
 class ConvertActionTests: XCTestCase {
-    // TODO: Change this back to `#if !os(iOS)` once https://github.com/apple/swift-corelibs-foundation/issues/5028 is resolved
-    #if os(macOS)
+    #if !os(iOS)
     let imageFile = Bundle.module.url(
         forResource: "TestBundle", withExtension: "docc", subdirectory: "Test Bundles")!
         .appendingPathComponent("figure1.png")