Tests: incorrect line references reported on error

[larabr]

In tests, the line references given on error are off, due to two separate issues:

• since webpack does not emit source map files for TS, a custom plugin should be set: https://github.com/ryanclark/karma-webpack/issues/109#issuecomment-224961264
• even with the above fix, karma-webpack v5 has a bug that breaks the source map referencing: https://github.com/ryanclark/karma-webpack/issues/493. The proposed temporary solution (set splitChunks: false) does not work for us, because chunking is needed in the tests to load the mailparses and openpgp.js. Their next karma-webpack release will hopefully fix the problem.