Switched CI to GH Actions

[slawekjaranowski]

fix: #15

• gradle wrapper was refreshed
• it is copied from mockito
• single job was moved to separate stage - verify
• check reproducibility require change in build - will be done by issue Make build reproducibility #18
• release stage also require change in build - will be done by issue Migrate to new Shipkit libraries #14

[skabashnyuk]

Hello. Is this a copy of https://github.com/mockito/mockito/tree/release/3.x/.github/workflows ? is anything extra here? Can you list it in the PR description?

[slawekjaranowski]

@mockitoguy @skabashnyuk Kindly reminder about it.
I will take care about next issues.

[skabashnyuk]

@slawekjaranowski May I ask you to take a look at build results? https://github.com/mockito/mockito-testng/actions/runs/384758526

[slawekjaranowski]

Please re-run github action problem ... on build (11, windows-latest)

Error: Exception in thread "main" java.io.IOException: Downloading from https://services.gradle.org/distributions/gradle-6.7.1-bin.zip failed: timeout

https://github.com/mockito/mockito-testng/runs/1457653160?check_suite_focus=true