Dispose of FileStream after TUS upload

The library doesn't do this for us. This would be unsafe if we weren't
awaiting the result of TusPatchAsync, but by the time TusPatchAsync
returns, the FileStream is no longer needed.

src/LfMerge.Core.Tests/SRTestEnvironment.cs

@@ -178,7 +178,7 @@ public async Task TusUpload(Uri tusEndpoint, string path, string mimeType)
 			var createResponse = await Http.TusCreateAsync(createOpts);
 
 			// That doesn't actually upload the file; TusPatchAsync does the actual upload
-			var fileStream = new FileStream(file.FullName, FileMode.Open, FileAccess.Read);
+			using var fileStream = new FileStream(file.FullName, FileMode.Open, FileAccess.Read);
 			var patchOpts = new TusPatchRequestOption {
 				FileLocation = createResponse.FileLocation,
 				Stream = fileStream,