Add package tsconfig-paths and integrate it into ts-test script

Ts-node does not load the paths from tsconfig.json by itself. Thus in
the tests run with ts-node the custom types for external packages (and
things like our offspring) can't be resolved.
The package tsconfig-paths has resulted from a discussion about this
topic on
github (TypeStrong/ts-node#138 (comment)),
which loads said modules.

package.json

@@ -195,12 +195,13 @@
     "supertest": "4.0.2",
     "terminal-img": "1.1.1",
     "ts-node": "8.3.0",
+    "tsconfig-paths": "3.8.0",
     "twitter": "1.7.1",
     "typescript": "3.5.3"
   },
   "scripts": {
     "ts-build": "npx tsc",
-    "ts-test": "npx mocha --async-only --bail --require ts-node/register --recursive --ui tdd $(find test -name '*.ts' ! -path '*network*')",
+    "ts-test": "npx mocha --async-only --bail --require ts-node/register --require tsconfig-paths/register --recursive --ui tdd $(find test -name '*.ts' ! -path '*network*')",
     "build": "run-script-os",
     "build:darwin:linux": "node bin/build.js --color",
     "build:win32": "node bin\\build.js --color",