fix(tests): replace gocovmerge with a simple bash script

scripts/gocovmerge.sh

@@ -3,7 +3,12 @@
 set -o errexit
 set -o pipefail
 
-gocovmerge profile.* | grep -vE '^github.com/prymitive/karma/cmd/karma/bindata_assetfs.go:' > coverage.txt
+echo "mode: set" > coverage.txt
+cat profile.* \
+  | grep -v mode: \
+  | grep -vE '^github.com/prymitive/karma/cmd/karma/bindata_assetfs.go:' \
+  | sort -r \
+  | awk '{if($1 != last) {print $0;last=$1}}' >> coverage.txt
 rm -f profile.*
 
 go tool cover -func coverage.txt | tail -n 1 | awk '{print $3}'