perf(transformer): leverage the ts-jest config set cache (#1318)

Instead of creating ConfigSet instances all the time (which is an expensive operation, as it involves path concatenations, merging configurations, reading from disk...), we use a private method in `ts-jest` that caches the configurations.

Fixes #1311

src/ng-jest-transformer.spec.ts

@@ -105,6 +105,7 @@ describe('NgJestTransformer', () => {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const transformSyncMock = transformSync as unknown as jest.MockInstance<unknown, any>;
     const transformCfg = {
+      cacheFS: new Map(),
       config: {
         cwd: process.cwd(),
         extensionsToTreatAsEsm: [],
@@ -161,6 +162,7 @@ describe('NgJestTransformer', () => {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const transformSyncMock = transformSync as unknown as jest.MockInstance<unknown, any>;
     const transformCfg = {
+      cacheFS: new Map(),
       config: {
         cwd: process.cwd(),
         extensionsToTreatAsEsm: [],

src/ng-jest-transformer.ts

@@ -50,7 +50,8 @@ export class NgJestTransformer extends TsJestTransformer {
     filePath: Config.Path,
     transformOptions: TransformOptionsTsJest,
   ): TransformedSource | string {
-    const configSet = this._createConfigSet(transformOptions.config);
+    // @ts-expect-error we are accessing the private cache to avoid creating new objects all the time
+    const configSet = super._configsFor(transformOptions);
     /**
      * TypeScript < 4.5 doesn't support compiling `.mjs` file by default when running `tsc` which throws error. Also we
      * transform `js` files from `node_modules` assuming that `node_modules` contains compiled files to speed up compilation.