coverage: add tests for arguments normalization

# Why

`foreground-child` has a complex signature but not all its variants are covered by tests.

# What

Add tests for the signatures not covered by the other tests.

test/normalize-fg-args.js

@@ -0,0 +1,96 @@
+var fg = require('../index.js')
+var spawn = require('child_process').spawn
+
+switch (process.argv[2]) {
+  case "child":
+    childMain();
+    break;
+  case "parent":
+    parentMain();
+    break;
+  case undefined:
+    test();
+    break;
+}
+
+function childMain() {
+  console.log(JSON.stringify(process.argv.slice(3)));
+}
+
+function parentMain() {
+  const fgArgs = JSON.parse(process.argv[3]);
+  fg.apply(null, fgArgs);
+}
+
+function test () {
+  var t = require('tap')
+
+  t.test('fg(process.execPath, [__filename, "child"])', function (t) {
+    t.plan(3);
+    var fgArgs = JSON.stringify([process.execPath, [__filename, "child"]]);
+    var args = [__filename, "parent", fgArgs];
+    var child = spawn(process.execPath, args);
+    var chunks = [];
+    child.stdout.on("data", function(chunk) {chunks.push(chunk)});
+    child.once("close", function(code, signal) {
+      t.equal(code, 0)
+      t.equal(signal, null)
+      var actual = JSON.parse(Buffer.concat(chunks).toString("UTF-8"));
+      var expected = [];
+      t.match(actual, expected)
+    });
+  });
+
+  t.test('fg(process.execPath, [__filename, "child", "foo"])', function (t) {
+    t.plan(3);
+    var fgArgs = JSON.stringify([process.execPath, [__filename, "child", "foo"]]);
+    var args = [__filename, "parent", fgArgs];
+    var child = spawn(process.execPath, args);
+    var chunks = [];
+    child.stdout.on("data", function(chunk) {chunks.push(chunk)});
+    child.once("close", function(code, signal) {
+      t.equal(code, 0)
+      t.equal(signal, null)
+      var actual = JSON.parse(Buffer.concat(chunks).toString("UTF-8"));
+      var expected = ["foo"];
+      t.match(actual, expected)
+    });
+  });
+
+  t.test('fg([process.execPath, __filename, "child", "bar"])', function (t) {
+    t.plan(3);
+    var fgArgs = JSON.stringify([[process.execPath, __filename, "child", "bar"]]);
+    var args = [__filename, "parent", fgArgs];
+    var child = spawn(process.execPath, args);
+    var chunks = [];
+    child.stdout.on("data", function(chunk) {chunks.push(chunk)});
+    child.once("close", function(code, signal) {
+      t.equal(code, 0)
+      t.equal(signal, null)
+      const actual = JSON.parse(Buffer.concat(chunks).toString("UTF-8"));
+      const expected = ["bar"];
+      t.match(actual, expected)
+    });
+  });
+
+  t.test('fg(process.execPath, __filename, "child", "baz")', function (t) {
+    t.plan(3);
+    const fgArgs = JSON.stringify([process.execPath, __filename, "child", "baz"]);
+    const args = [__filename, "parent", fgArgs];
+    const child = spawn(process.execPath, args);
+    const chunks = [];
+    child.stdout.on("data", function(chunk) {chunks.push(chunk)});
+    const err = [];
+    child.stderr.on("data", function(chunk) {err.push(chunk)});
+    child.once("close", function(code, signal) {
+      t.equal(code, 0)
+      t.equal(signal, null)
+      var errs = JSON.parse(Buffer.concat(err).toString("UTF-8"));
+      var actual = JSON.parse(Buffer.concat(chunks).toString("UTF-8"));
+      var expected = ["baz"];
+      t.match(actual, expected)
+    });
+  });
+
+  t.end()
+}