add unit test to cover new code

Author: infeo

src/test/java/org/cryptomator/cryptofs/DirectoryIdBackupTest.java

@@ -97,6 +97,13 @@ public void beforeEachRead() throws IOException {
 			decChannel = mock(DecryptingReadableByteChannel.class);
 		}
 
+		@Test
+		@DisplayName("If the given path is not a cipherContentDir, throw IllegalArgumentException")
+		public void wrongPath() throws IOException {
+			var dirIdBackupSpy = spy(dirIdBackup);
+			Assertions.assertThrows(IllegalArgumentException.class, () -> dirIdBackupSpy.read(testDir));
+		}
+
 		@Test
 		@DisplayName("If the directory id is longer than 36 characters, throw IllegalStateException")
 		public void contentLongerThan36Chars() throws IOException {