test: replace [].join() with ''.repeat()

Replace [].join calls with the more modern .repeat

PR-URL: #12305
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

test/parallel/test-child-process-send-utf8.js

@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const fork = require('child_process').fork;
 
-const expected = Array(1e5).join('ßßßß');
+const expected = 'ßßßß'.repeat(1e5 - 1);
 if (process.argv[2] === 'child') {
   process.send(expected);
 } else {

test/parallel/test-stringbytes-external.js

@@ -23,9 +23,9 @@ assert.strictEqual(b[1], 0);
 assert.strictEqual(ucs2_control, c);
 
 // now create big strings
-const size = 1 + (1 << 20);
-write_str = Array(size).join(write_str);
-ucs2_control = Array(size).join(ucs2_control);
+const size = 1 << 20;
+write_str = write_str.repeat(size);
+ucs2_control = ucs2_control.repeat(size);
 
 // check resultant buffer and output string
 b = Buffer.from(write_str, 'ucs2');
@@ -110,7 +110,7 @@ const PRE_3OF4_APEX = Math.ceil((EXTERN_APEX / 4) * 3) - RADIOS;
 
 // https://github.com/nodejs/node/issues/1024
 {
-  const a = Array(1 << 20).join('x');
+  const a = 'x'.repeat(1 << 20 - 1);
   const b = Buffer.from(a, 'ucs2').toString('ucs2');
   const c = Buffer.from(b, 'utf8').toString('utf8');