Add developer docs

docs/dev-testing.md

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+## Overview
+
+We use the standard golang testing environment.
+
+The following conventions are used:
+
+* *White box unit tests*: Tests for internal functionality are placed in files
+* *Black box unit tests*: Tests for public interfaces are placed in files
+with `<package name>_test.go` and belong to the package `<package_name>_test`.
+There only exists one package test file per package.
+* *Integration tests*: Tests that use multiple componenents are placed in a
+package test file. These are named `<package name>_test.go` and belong to the
+package `<package_name>_test`.
+* *Test assets*: Any required files are placed in a directory `./testdata`
+within each package directory.
+
+## Executing tests
+
+Visual Studio Code has a very good golang test integration.
+For debugging a test this is the recommended solution.
+
+The Makefile provided by us has a `test` target that executes:
+```
+$ go clean -testcache
+$ go build ./...
+$ go vet ./...
+$ go test ./...
+```
+
+Of course the commands can also be used on the command line.
+For details about golang testing refer to the standard documentation:
+
+* [Testing package](https://pkg.go.dev/testing)
+* [go test command](https://pkg.go.dev/cmd/go#hdr-Test_packages)