use Go 1.20 support for coverage testing of programs

[majewsky]

Ref: https://go.dev/doc/go1.20

Go 1.20 supports collecting code coverage profiles for programs (applications and integration tests), as opposed to just unit tests.

To collect coverage data for a program, build it with go build's -cover flag, then run the resulting binary with the environment variable GOCOVERDIR set to an output directory for coverage profiles.