refactor: move testUtils out of src

[overbalance]

What problem is this solving?

Ensure the src folder has only SDK code and associated tests.

Short description of changes

• Moved test utility files from src/testUtils/ to tests/utils/
• Flattened subdirectory structure (e.g., FailingStorage/index.ts → FailingStorage.ts)
• Renamed fakeFetch functions to use fakeFetch prefix (e.g., getBody → fakeFetchGetBody)
• Updated all import paths in test files to reference the relocated utilities

Testing

• CI and local suites pass

[github-actions]

Chrome DevTools Protocol Tracing (Script: 96.47ms, Heap: 13.49MB)

	Number of Requests	Size of Requests	Script Duration	Task Duration	Heap Used Size
Requests	+3 requests	+29.78 KB
Page Loaded			+15.59 ms	+8.21 ms	+0.94 MB
Generate 100 fetch requests			+14.69 ms	+64.10 ms	+1.73 MB
Generate 100 XHR requests			+32.89 ms	+86.25 ms	+2.31 MB
Click 100 buttons and generate 100 logs			+26.82 ms	+34.68 ms	+2.93 MB
Throw a 100 exceptions			-0.49 ms	+7.11 ms	+2.46 MB
End Session			+6.98 ms	+13.32 ms	+3.13 MB
Total	+3 requests	+29.78 KB	+96.47 ms	+213.67 ms	+13.49 MB

Lighthouse (Script Eval: 45.89ms)

	Difference	Description
Total Blocking Time	0 ms	Difference in Total Blocking Time: Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds. Learn more about the Total Blocking Time metric.
Main Thread Time	+58.70 ms	Difference in Main Thread Time: Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to minimize main-thread work
Script Evaluation Time	+45.89 ms	Difference in Script Evaluation Time: Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to reduce Javascript execution time.

Platform Tests (vite-7 gzip: 47.60KB)

vite-7 Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-7 - esnext	+163.90 KB	+47.60 KB
vite-7 - es2015	+170.67 KB	+49.25 KB

vite-otel-latest Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-otel-latest - esnext	+162.97 KB	+49.11 KB
vite-otel-latest - es2015	+169.67 KB	+50.70 KB

webpack-5 Platform Tests

	Total Uncompressed Size	Total Gzip Size
webpack-5 - esnext	+120.06 KB	+42.58 KB
webpack-5 - es2015	+121.18 KB	+44.09 KB