Tests: adjust LMDB test harness for Windows

Ensure that we release the open handles before attempting to clean up
the temporary directory.  This repairs the execution of 11 tests on
Windows, bringing us to a 95% pass rate (excluding `LogHandleTests`).

Author: compnerd

Tests/SwiftDocCTests/Utility/LMDBTests.swift

@@ -20,6 +20,16 @@ final class SwiftLMDBTests: XCTestCase {
         let tempURL = try createTemporaryDirectory()
 
         environment = try! LMDB.Environment(path: tempURL.path, maxDBs: 4, mapSize: 1024 * 1024 * 1024) // 1GB of mapSize
+
+        // `addTeardownBlock` execute in LIFO order, which is important as we
+        // need this to execute before the implicit closure for the cleaning of
+        // the temporary directory.
+        addTeardownBlock {
+            // Explicitly close the environment to release the handles to the
+            // database files.  This is required on Windows where the files
+            // cannot be deleted while they are open.
+            self.environment.close()
+        }
     }
 
     override func tearDown() {