Internal tooling improvements 2020Q3

[bajtos]

Improve the internal tooling we use to develop LoopBack 4 (2020Q3 edition).

• Enable parallel mocha testing feat: enable parallel mocha testing #5011
• Move slow tests from npm test to a different test task Move slow tests from npm test to a different test task #5670
• Spike: Switch from lerna to pnpm workspaces Spike: Switch from lerna to pnpm workspaces #5671

[marioestradarosa]

@bajtos Does mocha has now support for parallel testings ?

[raymondfeng]

@marioestradarosa We have #5011. It will be retested and landed once a new release of Mocha is published.

[dhmlau]

Closing for Q3 scope.