This commit gets ready for the introduction of ngtsc template
type-checking tests by refactoring test environment setup into a
custom helper. This helper will simplify the authoring of future
ngtsc tests.
Ngtsc tests previously returned a numeric error code (a la ngtsc's CLI
interface) if any TypeScript errors occurred. The helper has the
ability to run ngtsc and return the actual array of ts.Diagnostics, which
greatly increases the ability to write clean tests.
PR Close#26203
Alex Rickabaugh