Fix test execution and update README setup steps

[bhokaremoin]

Summary

• Fixes test execution in the Java Playwright Percy example by ensuring Maven Surefire runs TestNG suites correctly.
• Updates README with correct setup steps and simplified flow based on documentation team feedback.

Changes

Test execution fix

• Removed junit-jupiter-api dependency from pom.xml.
• Added testng under maven-surefire-plugin dependencies to force TestNG provider usage.

README updates

• Added npx playwright install command to Step 1 for downloading browser binaries.
• Reworded Step 4 as "Generate first Percy build (baseline)" with clearer descriptions.
• Simplified Step 5 to "Generate second Percy build (with visual changes)" — removed manual code modification instructions and dashboard approval step. Users just run the "after" test which has pre-made visual changes.
• Added node_modules/ to .gitignore.

Why

• Surefire was auto-selecting JUnit Platform, resulting in Tests run: 0 for configured TestNG suite files.
• README was outdated — missing playwright install step, and had unnecessary manual steps for generating the second build.

Validation

• Ran: npx percy exec --verbose -- mvn test -P web-percy-test
• Result: Test suite executes correctly (Tests run: 1)

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