Improve test coverage of CustomTestStringConvertible.swift. (#628)

This PR adds 100% test coverage to CustomTestStringConvertible.swift.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Diff for: Tests/TestingTests/CustomTestStringConvertibleTests.swift

@@ -0,0 +1,79 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+@testable @_spi(Experimental) @_spi(ForToolsIntegrationOnly) import Testing
+
+private import _TestingInternals
+
+@Suite("CustomTestStringConvertible Tests")
+struct CustomTestStringConvertibleTests {
+  @Test func optionals() {
+    #expect(String(describingForTest: 0 as Int?) == "0")
+    #expect(String(describingForTest: "abc" as String?) == #""abc""#)
+    #expect(String(describingForTest: nil as Int?) == "nil")
+    #expect(String(describingForTest: nil as String?) == "nil")
+    #expect(String(describingForTest: nil as _OptionalNilComparisonType) == "nil")
+  }
+
+  @Test func strings() {
+    #expect(String(describingForTest: "abc") == #""abc""#)
+    #expect(String(describingForTest: "abc"[...] as Substring) == #""abc""#)
+  }
+
+  @Test func ranges() {
+    #expect(String(describingForTest: 0 ... 1) == "0 ... 1")
+    #expect(String(describingForTest: 0...) == "0...")
+    #expect(String(describingForTest: ...1) == "...1")
+    #expect(String(describingForTest: ..<1) == "..<1")
+    #expect(String(describingForTest: 0 ..< 1) == "0 ..< 1")
+  }
+
+  @Test func types() {
+    #expect(String(describingForTest: Self.self) == "CustomTestStringConvertibleTests")
+    #expect(String(describingForTest: NonCopyableType.self) == "NonCopyableType")
+  }
+
+  @Test func enumerations() {
+    #expect(String(describingForTest: SWTTestEnumeration.A) == "SWTTestEnumeration(rawValue: \(SWTTestEnumeration.A.rawValue))")
+    #expect(String(describingForTest: SomeEnum.elitSedDoEiusmod) == ".elitSedDoEiusmod")
+  }
+
+  @Test func otherProtocols() {
+    #expect(String(describingForTest: CustomStringConvertibleType()) == "Lorem ipsum")
+    #expect(String(describingForTest: TextOutputStreamableType()) == "Dolor sit amet")
+    #expect(String(describingForTest: CustomDebugStringConvertibleType()) == "Consectetur adipiscing")
+  }
+}
+
+// MARK: - Fixtures
+
+private struct NonCopyableType: ~Copyable {}
+
+private struct CustomStringConvertibleType: CustomStringConvertible {
+  var description: String {
+    "Lorem ipsum"
+  }
+}
+
+private struct TextOutputStreamableType: TextOutputStreamable {
+  func write(to target: inout some TextOutputStream) {
+    target.write("Dolor sit amet")
+  }
+}
+
+private struct CustomDebugStringConvertibleType: CustomDebugStringConvertible {
+  var debugDescription: String {
+    "Consectetur adipiscing"
+  }
+}
+
+private enum SomeEnum {
+  case elitSedDoEiusmod
+}

Diff for: Tests/TestingTests/TypeInfoTests.swift

@@ -76,4 +76,4 @@ extension String {
   enum NestedType {}
 }
 
-enum SomeEnum {}
+private enum SomeEnum {}