chore: add test for cached res timings

[vigneshshanmugam]

• fix Content download wrong for cached requests uptime#460
• The issue itself was fixed by moving to the network events from PW instead of CDP logic feat: use network events from pw #488
• Added test case now for timings for the cached resource check.

Steps to check

1. Create inline journey with the example given on the issue.

step("My Page", async () => {
   await page.goto('https://paulb-elastic.github.io/testPageWithCounter.html');
   await page.waitForSelector('#counter >> text=Seconds since page loaded: 2');
});
step("Cached version of My Page", async () => {
   await page.goto('https://paulb-elastic.github.io/testPageWithCounter.html');
   await page.waitForSelector('#counter >> text=Seconds since page loaded: 2');
});

2. Run the synthetics CLI from local and ensure the timing data is correct

cat examples/inline/example.js | node dist/cli.js --inline --rich-events

3. Install the current synthetics version globally npm install -g .and run it as inline monitor.

  - type: browser
    id: cache-test
    name: Cached-steps
    schedule: "@every 1m"
    source:
      inline:
        script: step("My Page", async () => {
          await page.goto("https://paulb-elastic.github.io/testPageWithCounter.html");
          });
          step("Cached version of My Page", async () => {
          await page.goto('https://paulb-elastic.github.io/testPageWithCounter.html');
          });

[vigneshshanmugam]

redundant, we wait until the idle part anyway in the previous line.

[apmmachine]

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• Start Time: 2022-04-12T19:04:28.225+0000

• Duration: 15 min 51 sec

Test stats 🧪

Test	Results
Failed	0
Passed	149
Skipped	2
Total	151

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[justinkambic]

LGTM, CI is passing, test looks good