Fix build failures on iOS (#363)

Make `temporaryDirectory()` available on non-macOS, non-Linux and
non-Windows platforms too.

### Motivation:

Currently our test fail to build for non-macOS, non-Linux and
non-Windows platforms.

### Modifications:

I moved `temporaryDirectory()` outside the conditional that made it only
available to macOS, Linux and Windows, but still inside the `#if
!SWT_NO_FILE_IO` condition.

### Result:

Our unit tests compile.

### 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: dennisweissmann

Tests/TestingTests/Support/FileHandleTests.swift

@@ -164,27 +164,6 @@ struct FileHandleTests {
 
 // MARK: - Fixtures
 
-func temporaryDirectory() throws -> String {
-#if SWT_TARGET_OS_APPLE
-  try withUnsafeTemporaryAllocation(of: CChar.self, capacity: Int(PATH_MAX)) { buffer in
-    if 0 != confstr(_CS_DARWIN_USER_TEMP_DIR, buffer.baseAddress, buffer.count) {
-      return String(cString: buffer.baseAddress!)
-    }
-    return try #require(Environment.variable(named: "TMPDIR"))
-  }
-#elseif os(Linux)
-  "/tmp"
-#elseif os(Windows)
-  try withUnsafeTemporaryAllocation(of: wchar_t.self, capacity: Int(MAX_PATH + 1)) { buffer in
-    // NOTE: GetTempPath2W() was introduced in Windows 10 Build 20348.
-    if 0 == GetTempPathW(DWORD(buffer.count), buffer.baseAddress) {
-      throw Win32Error(rawValue: GetLastError())
-    }
-    return try #require(String.decodeCString(buffer.baseAddress, as: UTF16.self)?.result)
-  }
-#endif
-}
-
 func withTemporaryPath<R>(_ body: (_ path: String) throws -> R) throws -> R {
   // NOTE: we are not trying to test mkstemp() here. We are trying to test the
   // capacity of FileHandle to open a file for reading or writing and we need a
@@ -226,4 +205,26 @@ extension FileHandle {
   }
 }
 #endif
+
+func temporaryDirectory() throws -> String {
+#if SWT_TARGET_OS_APPLE
+  try withUnsafeTemporaryAllocation(of: CChar.self, capacity: Int(PATH_MAX)) { buffer in
+    if 0 != confstr(_CS_DARWIN_USER_TEMP_DIR, buffer.baseAddress, buffer.count) {
+      return String(cString: buffer.baseAddress!)
+    }
+    return try #require(Environment.variable(named: "TMPDIR"))
+  }
+#elseif os(Linux)
+  "/tmp"
+#elseif os(Windows)
+  try withUnsafeTemporaryAllocation(of: wchar_t.self, capacity: Int(MAX_PATH + 1)) { buffer in
+    // NOTE: GetTempPath2W() was introduced in Windows 10 Build 20348.
+    if 0 == GetTempPathW(DWORD(buffer.count), buffer.baseAddress) {
+      throw Win32Error(rawValue: GetLastError())
+    }
+    return try #require(String.decodeCString(buffer.baseAddress, as: UTF16.self)?.result)
+  }
+#endif
+}
+
 #endif