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* [skip changelog] Sync testing infrastructure with "template" assets

We have assembled a collection of reusable project assets:
https://github.com/arduino/tooling-project-assets
These will be used in the repositories of all Arduino tooling projects.

Some minor improvements and standardizations have been made in the upstream "template" assets, and those are hereby
introduced to this repository.

Notable:

- Configure paths filters to avoid unnecessary workflow runs
- Increased parallelism
- Improved maintainability

* Remove obsolete testing tasks

The sync with the template testing assets has resulted in new standardized names for some of the test runner tasks:

- `test-unit` -> `go:test`
- `test-integration` -> `go:test-integration`

* [skip changelog] Update package names in imports

As of Go 1.7 the `golang.org/x/net/context` package is available in the standard library under the name `context`.

* [skip changelog] Tidy root module dependencies

Result of running `go mod tidy` with Go 1.16.6.

* [skip changelog] Tidy `github.com/arduino/arduino-cli/arduino/discovery/discovery_client` module's dependencies

Result of running `go mod tidy` with Go 1.16.6.

* [skip changelog] Tidy `github.com/arduino/arduino-cli/term_example` module's dependencies

Result of running `go mod tidy` with Go 1.16.6.

* [skip changelog] Tidy `github.com/arduino/arduino-cli/docsgen` module's dependencies

Result of running `go mod tidy` with Go 1.16.6.