[test] Exclude repository root from assertions

[eps1lon]

The repository root was hidden in string length assertions. Those broke once you tested React deeper in your filesystem.

Added the repo root behind a global __REACT_ROOT_PATH_TEST__ since that's used in a couple of places. Defining that for each test makes it annoying to move test file around.

[react-sizebot]

Comparing: b061b59...da6c9b1

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	608.16 kB	608.16 kB	=	107.67 kB	107.67 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	667.39 kB	667.39 kB	=	117.55 kB	117.55 kB
facebook-www/ReactDOM-prod.classic.js	=	693.47 kB	693.47 kB	=	122.05 kB	122.05 kB
facebook-www/ReactDOM-prod.modern.js	=	683.89 kB	683.89 kB	=	120.43 kB	120.43 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against da6c9b1