PathEncoderManagerTest Bug.

[stone-98]

Describe the bug

nacos/common/src/test/java/com/alibaba/nacos/common/pathencoder/PathEncoderManagerTest.java

Lines 86 to 91 in c4aac2b

	@Test
	public void testEncodeWithNonExistOs() {
	System.setProperty("os.name", "non-exist");
	String testCase = "aa||a";
	Assert.assertEquals(testCase, PathEncoderManager.getInstance().encode(testCase));
	}

System.setProperty("os.name", "non-exist");设置无效，导致unit test报错，如下：

原因：PathEncoderManager采用饿汉单例模式。调整为采用反射进行test。