Previously, `R3TestBedCompiler` was dynamically defining an
`@NgModule`-decorated `CompilerModule` class inside a method call.
Since ngcc only processes top-level classes, this class was not
transformed causing failures in unit tests (see #30121 for details).
This commit fixes it by using `compileNgModuleDefs()` directly (similar
to the fix in #30037).
Fixes#30121
PR Close#28530