Disable parallel test discovery; we already use Grade workers for max parallelism

[arturbosch]

Additional thread parallelism in a Gradle worker does not increase the test throughput:

with spek2.discovery.parallel.enabled=1
https://scans.gradle.com/s/wf5ehygo374ck/tests?toggled=WyI6ZGV0ZWt0LWdyYWRsZS1wbHVnaW4iLCI6ZGV0ZWt0LXJ1bGVzLWVycm9ycHJvbmUiLCI6ZGV0ZWt0LXJ1bGVzLXN0eWxlIiwiOmRldGVrdC1ydWxlcy1jb21wbGV4aXR5Il0

spek2.discovery.parallel.enabled=0
https://scans.gradle.com/s/kpvph5aoudc5c/tests?toggled=WyI6ZGV0ZWt0LWdyYWRsZS1wbHVnaW4iLCI6ZGV0ZWt0LXJ1bGVzLXN0eWxlIiwiOmRldGVrdC1ydWxlcy1lcnJvcnByb25lIl0

[codecov]

Codecov Report

Merging #3387 (bc62742) into master (f9e3e41) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3387   +/-   ##
=========================================
  Coverage     80.43%   80.43%           
  Complexity     2740     2740           
=========================================
  Files           448      448           
  Lines          8281     8281           
  Branches       1573     1573           
=========================================
  Hits           6661     6661           
  Misses          772      772           
  Partials        848      848           

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