Jest tests must be defined synchronously

Remove `async` in the describe tests definition.
Resolve the following warning:

> Returning a Promise from "describe" is not supported. Tests must be defined synchronously.
> Returning a value from "describe" will fail the test in a future version of Jest.
1 job for reduce-tests-warnings in 3 minutes and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
failed #497
npm-test

00:03:06

 
Name Stage Failure
failed
npm-test Build
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the pagedjs@0.1.36 specs script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /home/node/.npm/_logs/2020-02-14T22_42_04_030Z-debug.log
npm ERR! Test failed. See above for more details.
ERROR: Job failed: exit status 1