Add findUnusedName to file_ops.js

Add function to find an unused name in a directory to lib/file_ops and
use it for finding an unused instance directory.

test/lib/file_ops.js

@@ -13,6 +13,13 @@ describe("lib/file_ops", function() {
 
 		await fs.outputFile(path.join(baseDir, "test", "file.txt"), "contents");
 		await fs.outputFile(path.join(baseDir, "test", "another file.txt"), "more contents");
+
+		await fs.outputFile(path.join(baseDir, "find", "file"), "contents");
+		await fs.outputFile(path.join(baseDir, "find", "file.txt"), "contents");
+		await fs.outputFile(path.join(baseDir, "find", "foo-1"), "contents");
+		await fs.outputFile(path.join(baseDir, "find", "foo-2"), "contents");
+		await fs.outputFile(path.join(baseDir, "find", "bar-1.txt"), "contents");
+		await fs.outputFile(path.join(baseDir, "find", "bar-2.txt"), "contents");
 	}
 
 	before(setupTestingEnv);
@@ -31,4 +38,32 @@ describe("lib/file_ops", function() {
 			assert.equal(newest, null);
 		});
 	});
+
+	describe("findUnusedName()", function() {
+		it("should return named unchanged if it does not exist", async function() {
+			let cases = [
+				[["file"], "file"],
+				[["file", ".txt"], "file.txt"],
+				[["file.txt", ".txt"], "file.txt"],
+			];
+			for (let [args, expected] of cases) {
+				let actual = await libFileOps.findUnusedName(path.join(baseDir, "test", "folder"), ...args);
+				assert.equal(actual, expected);
+			}
+		});
+		it("should return changed name if it does exist", async function() {
+			let cases = [
+				[["file"], "file-2"],
+				[["file", ".txt"], "file-2.txt"],
+				[["file.txt", ".txt"], "file-2.txt"],
+				[["foo-1"], "foo-3"],
+				[["bar-1", ".txt"], "bar-3.txt"],
+				[["bar-1.txt", ".txt"], "bar-3.txt"],
+			];
+			for (let [args, expected] of cases) {
+				let actual = await libFileOps.findUnusedName(path.join(baseDir, "find"), ...args);
+				assert.equal(actual, expected);
+			}
+		});
+	});
 });