test: fix coverage coverage accounting

Commit "github-ci: add Coveralls support"
(31ebde8) introduced code coverage
support. However there is wrong value for option -covermode was
specified - "count". According to documentation [1] "count" calculates
how many times did each statement run and "atomic" mode is like "count",
but counts precisely in parallel programs. We don't use parallel mode in
tests at all (-p 1) and for code coverage measurement particularly, but
it is better using "atomic" mode instead "count".

Option "-coverpkg" was missed. "coverpkg" applies coverage analysis in
each test to packages matching the patterns. The default is for each
test to analyze only the package being tested. Total statement coverage
without -coverpkg is 66.8% and with -coverpkg is 69.3% according to "go
tool cover".

1. https://go.dev/blog/cover

Author: ligurio

Makefile

@@ -62,7 +62,7 @@ test-main:
 coverage:
 	go clean -testcache
 	go get golang.org/x/tools/cmd/cover
-	go test ./... -v -p 1 -covermode=count -coverprofile=$(COVERAGE_FILE)
+	go test ./... -v -p 1 -covermode=atomic -coverprofile=$(COVERAGE_FILE) -coverpkg=./...
 
 .PHONY: coveralls
 coveralls: coverage