test(ngcc): don't use the workspace typescript version to generate integration test fixtures (angular#44448)

Now that ViewEngine libraries can no longer be created, the latest
TypeScript version that ngcc should be able to process is TypeScript 4.3,
i.e. the version of TypeScript that was supported in Angular 12.
However, ngcc's integration tests used the TypeScript version of the
workspace to create the JavaScript files from TypeScript sources on
demand. This introduces friction when upgrading the TypeScript version
within the workspace, as changes to TypeScript's emit format may affect
ngcc's ability to process it correctly.

The on demand creation of JavaScript files was convenient for authoring
tests, but it also helped to detect incompatibilities with newer
versions of TypeScript. Now that ngcc no longer has to process newer
versions of TypeScript, we want to pin the integration suite to use
JavaScript code as if it were compiled using TypeScript 4.3, i.e. a
version of TypeScript that is actually supported by ngcc.

This commit updates the integration test to inline all the generated
files directly in the tests, instead of compiling them on demand. This
was done by temporarily installing TypeScript 4.3 and using it to create
the `loadTestFiles` statement from the original TypeScript inputs.

An alternative could have been to install TypeScript 4.3 as an actual
dependency in the workspace and using that to continue compiling the
integration suite on demand, but this brings some overhead in package
installations (TypeScript is ~60MB) and the authoring aspect of ngcc
integration test is expected to diminish, now that ngcc's support is no
longer a moving target.

PR Close angular#44448

Author: JoostK