Indexing: add a workaround for clearing the index (#647)

The unit tests will occasionally desire to clean up the generated index
while retaining a reference to the NavigatorIndex instance.  This is
prevented on Windows as the LMDB environment will preserve an open
handle which results in the index not being removed.

Co-authored-by: Kyle <kyle201817146@gmail.com>

Author: compnerd

Tests/SwiftDocCUtilitiesTests/ConvertActionTests.swift

@@ -1981,6 +1981,7 @@ class ConvertActionTests: XCTestCase {
         let indexFromConvertAction = try NavigatorIndex.readNavigatorIndex(url: indexURL)
         XCTAssertEqual(indexFromConvertAction.count, 37)
 
+        indexFromConvertAction.environment?.close()
         try FileManager.default.removeItem(at: indexURL)
 
         // Run just the index command over the built documentation