I'm trying to test my LitElement components with Karma but importing PDF.JS is causing the test file not to be loaded.

Example of the LitElement component file

import * as pdfjsLib from 'pdfjs-dist/build/pdf.js';
import * as pdfjsWorker from 'pdfjs-dist/build/pdf.worker.entry.js';
import * as pdfjsSandbox from 'pdfjs-dist/build/pdf.sandbox.js';
import * as pdfjsViewer from 'pdfjs-dist/web/pdf_viewer.js';
import pdfjsViewerCss from 'pdfjs-dist/web/pdf_viewer.css';

export class UiDocReviewContext extends ScopedElementsMixin(LitElement) {
  static get properties() {
    return {
        ....

Example of the Karma test file

import '../ui-doc-review-context.js';

describe('feat-upload', async () => {
  beforeEach(async () => {
    await fixture(html`<ui-doc-review-context
      .data=${{}}
      .pageNum=${1}
      .box=${{}}
    ></ui-doc-review-context>`);
  });
})

The issue
Karma gives the following error:

Chrome Headless 100.0.4896.127 (Mac OS 10.15.7) ERROR: 'Error loading test file: /packages/ui-doc-review-context/test/ui-doc-review-context.test.js'
Chrome Headless 100.0.4896.127 (Mac OS 10.15.7) ERROR: 'TypeError: Failed to fetch dynamically imported module: http://localhost:9876/base/packages/ui-doc-review-context/test/ui-doc-review-context.test.js?c2a0b72b2b28b1f3a57be6918056fed53cfcd90c'

In the console logs I get this:

Failed to load module script: Expected a JavaScript module script but the server responded with a MIME type of "text/css". Strict MIME type checking is enforced for module scripts per HTML spec.
context.js:265 Error loading test file: /packages/ui-doc-review-context/test/ui-doc-review-context.test.js
context.js:265 TypeError: Failed to fetch dynamically imported module: http://localhost:9876/base/packages/ui-doc-review-context/test/ui-doc-review-context.test.js?c2a0b72b2b28b1f3a57be6918056fed53cfcd90c