Create 'security' directory test should try writing files too (swiftl…

…ang#3941)

Motivation:
Source compat test continues to fail even with swiftlang#3928:

```
"You don’t have permission to save the file “fingerprints” in the folder “security”
```

https://ci.swift.org/job/swift-PR-source-compat-suite/5709/artifact/

The tests we did with swiftlang#3938 shows that we can create directories but not write files.

Modifications:
Test creating `security` directory and writing file in it, and disable TOFU feature if the test fails.

Sources/Commands/SwiftTool.swift

@@ -648,6 +648,8 @@ public class SwiftTool {
             if !fileSystem.exists(sharedSecurityDirectory) {
                 try fileSystem.createDirectory(sharedSecurityDirectory, recursive: true)
             }
+            // And make sure we can write files (locking the directory writes a lock file)
+            try fileSystem.withLock(on: sharedSecurityDirectory, type: .exclusive) { }
             return sharedSecurityDirectory
         } catch {
             self.observabilityScope.emit(warning: "Failed creating shared security directory: \(error)")