migrate AtriumErrorSpec from spek to kotest

[robstoll]

Platform (all, jvm, js): all
Extension (none, kotlin 1.3): none

Code related feature

We want to move away from spek as it's future regarding maintenance and development is quite unsure.
As I don't want to loose hierarchical test setup I decided that we go with kotest

Following the things you need to do:

• add dependency to kotest in misc/atrium-specs/build.gradle.kts
• rewrite atrium-core/src/commonTest/kotlin/ch/tutteli/atrium/reporting/AtriumErrorSpec.kt to kotest extending a DescribeSpec

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[jakubriegel]

I'll work on this