SR-10512: TestFileManager .creationDate failed on Ubuntu 14.04

- .creationDate may not be available on all Linux platforms due to
  kernel version, Docker or other environment security settings etc
  so only check its value if it is non-nil.

- Re-enable this test but allow for .creationDate possibly being nil.

Author: spevans

Tests/Foundation/Tests/TestFileManager.swift

@@ -336,18 +336,10 @@ class TestFileManager : XCTestCase {
             XCTAssertNotNil(fileGroupOwnerAccountID)
 
 #if os(Linux)
-            /* ⚠️ */
-            if shouldAttemptXFailTests("Checking that .creationDate is set is failing on Ubuntu 16.04 when running from a Docker image. https://bugs.swift.org/browse/SR-10512") {
-                let requiredVersion = OperatingSystemVersion(majorVersion: 4, minorVersion: 11, patchVersion: 0)
-                let creationDate = attrs[.creationDate] as? Date
-                if ProcessInfo.processInfo.isOperatingSystemAtLeast(requiredVersion) {
-                    XCTAssertNotNil(creationDate)
-                    XCTAssertGreaterThan(Date().timeIntervalSince1970, try XCTUnwrap(creationDate).timeIntervalSince1970)
-                } else {
-                    XCTAssertNil(creationDate)
-                }
+            // .creationDate is not not available on all Linux systems but if it is supported check the value is reasonable
+            if let creationDate = attrs[.creationDate] as? Date {
+                XCTAssertGreaterThan(Date().timeIntervalSince1970, creationDate.timeIntervalSince1970)
             }
-            /* ⚠️ */ 
 #endif
 
             if let fileOwnerAccountName = attrs[.ownerAccountName] {