Add reason for failure in test

Author: spevans

TestFoundation/TestNSData.swift

@@ -946,7 +946,7 @@ extension TestNSData {
             let maps = try String(contentsOfFile: "/proc/self/maps", encoding: .utf8)
             XCTAssertTrue(maps.characters.count > 0)
         } catch {
-            XCTFail()
+            XCTFail("Cannot read /proc/self/maps: \(String(describing: error))")
         }
 #endif
     }