test: move timer-dependent test to sequential

Timer-dependent tests fail frequently on certain platforms in CI when
run in parallel with other tests, likely due to competition for
resources. Move test-repl-timeout-throw to sequential to avoid this
problem. Also did some minor refactoring (var->const and more use of
assert.strictEqual of looser assertions).

PR-URL: #9431
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergstrom <bugs@bergstroem.nu>

test/parallel/test-repl-timeout-throw.js → test/sequential/test-repl-timeout-throw.js

@@ -1,10 +1,10 @@
 'use strict';
 const common = require('../common');
-var assert = require('assert');
+const assert = require('assert');
 
-var spawn = require('child_process').spawn;
+const spawn = require('child_process').spawn;
 
-var child = spawn(process.execPath, [ '-i' ], {
+const child = spawn(process.execPath, [ '-i' ], {
   stdio: [null, null, 2]
 });
 
@@ -52,8 +52,8 @@ child.stdout.once('data', function() {
 });
 
 child.on('close', function(c) {
-  assert(!c);
+  assert.strictEqual(c, 0);
   // make sure we got 3 throws, in the end.
   var lastLine = stdout.trim().split(/\r?\n/).pop();
-  assert.equal(lastLine, '> 3');
+  assert.strictEqual(lastLine, '> 3');
 });