Migrate update-lock tests as integration tests

Update-lock tests now use the actual filesystem for testing, instead of
relying on stubs and spies.

This commit also fixes a small bug with update-lock that would cause a
`PromiseRejectionHandledWarning` when the lock callback would throw.

test/unit/lib/update-lock.spec.ts

@@ -0,0 +1,17 @@
+import { expect } from 'chai';
+import * as path from 'path';
+
+import * as updateLock from '~/lib/update-lock';
+
+describe('lib/update-lock: unit tests', () => {
+	describe('lockPath', () => {
+		it('should return path prefix of service lockfiles on host', () => {
+			expect(updateLock.lockPath(123)).to.equal(
+				path.join(updateLock.BASE_LOCK_DIR, '123'),
+			);
+			expect(updateLock.lockPath(123, 'main')).to.equal(
+				path.join(updateLock.BASE_LOCK_DIR, '123', 'main'),
+			);
+		});
+	});
+});