test: move test-fs-largefile to pummel

test-fs-largefile was disabled. It was fixed in bbf74fb but left in
disabled because it generates a 5Gb file. However, gibfahn had the
sensible suggestion of moving it to the pummel directory. Which is what
this change does.

In pummel, lint rules are applied, so this does necessitate changing a
pair of `var` declarations to `const`.

PR-URL: https://github.com/nodejs/node/pull/14338
Refs: https://github.com/nodejs/node/commit/bbf74fb87b2f8273e1d93e949f9c840b12ce4805
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/pummel/test-fs-largefile.js

@@ -35,9 +35,9 @@ const message = 'Large File';
 
 fs.truncateSync(fd, offset);
 assert.strictEqual(fs.statSync(filepath).size, offset);
-var writeBuf = Buffer.from(message);
+const writeBuf = Buffer.from(message);
 fs.writeSync(fd, writeBuf, 0, writeBuf.length, offset);
-var readBuf = Buffer.allocUnsafe(writeBuf.length);
+const readBuf = Buffer.allocUnsafe(writeBuf.length);
 fs.readSync(fd, readBuf, 0, readBuf.length, offset);
 assert.strictEqual(readBuf.toString(), message);
 fs.readSync(fd, readBuf, 0, 1, 0);