Get rid of warnings in console when running tests

[psmyrek]

This is the follow up of #160.

Running tests outputs a few lines of warnings in console:

(node:6080) Warning: Accessing non-existent property 'VERSION' of module exports inside circular dependency
(Use `node --trace-warnings ...` to show where the warning was created)

WARNING in ./tests/plugin/integration.js 53:19-33
"export 'createLocalVue' was not found in '@vue/test-utils'

WARNING in ./tests/_utils/vueadapter.js 7:19-36
"export 'default' (imported as 'VueModule') was not found in 'vue'

WARN: 'You are running the esm-bundler build of Vue. It is recommended to configure your bundler to explicitly replace feature flag 
globals with boolean literals to get proper tree-shaking in the final bundle. See http://link.vuejs.org/feature-flags for more details.'

Some of them (2 and 3) are caused because the codebase targets both Vue versions (v2.x and v3.x) and Vue has changed its exports and API between these versions.