Tweak playwright workers

[Tyriar]

I'm hoping this will reduce flakiness, in particular this issue that can happen sometimes:

1) [Firefox Stable] › ../../test/playwright/CharWidth.test.ts:20:9 › CharWidth Integration Tests › getStringCellWidth › ASCII chars 

    "beforeAll" hook timeout of 10000ms exceeded.

       8 |
       9 | let ctx: ITestContext;
    > 10 | test.beforeAll(async ({ browser }) => {
         |      ^
      11 |   ctx = await createTestContext(browser);
      12 |   await openTerminal(ctx);
      13 | });

        at /home/runner/work/xterm.js/xterm.js/test/playwright/CharWidth.test.ts:10:6

This also speeds up tests by using more cores on high core machines.