perf: use context isolation for inidividual journeys

[vigneshshanmugam]

• This PR improves the test execution time when there are multiple journeys involved while running the test suites. Previously we relied on spawning browser instance for every journey execution which is not required as context already provides enough isolation and each context is fresh (does not share any state or cache) used for each journey runs

• This removes the time that is required for spawning browser for each run which is in order of ~100ms.

Results (Approx)

1. For todos directory with 4 journeys

Before - 3600ms
After - 2400ms

2. Custom with increased number of journeys to showcase the impact.

// 20 journeys
Before - 11200ms
After - 5800ms

// 50 journeys
Before - 28700ms
After - 14800ms

[apmmachine]

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Build stats

• Build Cause: Pull request #274 opened

• Start Time: 2021-05-11T20:52:04.470+0000

• Duration: 15 min 56 sec

• Commit: e3a3db5

Test stats 🧪

Test	Results
Failed	0
Passed	87
Skipped	0
Total	87

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[andrewvc]

LGTM