Fixed Dir#close specs on Windows #1875

Job	Run time
RuboCop	31s
specs (ubuntu, 3.0.6)	1m 34s
specs (ubuntu, 3.1.4)	1m 31s
specs (ubuntu, 3.2.2)	1m 28s
specs (ubuntu, 3.3.0)	1m 27s
specs (macos, 3.0.6)	1m 32s
specs (macos, 3.1.4)	1m 35s
specs (macos, 3.2.2)	1m 22s
specs (macos, 3.3.0)	1m 10s
specs (windows, 3.0.6)	4m 15s
specs (windows, 3.1.4)	5m 6s
specs (windows, 3.2.2)	4m 27s
specs (windows, 3.3.0)	3m 58s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 26s
	31m 22s

Provide feedback