Edge-home-orchestration-go: Testing Guide Update

Testing Policy should have how to install coverage tool (gocov and gocov-html).

Error Log when coverage tools are not installed.

t25kim@t25kim:~/work/edge-home-orchestration-go$ make test
.
.
--------------------------------------
  build test for internal/...
--------------------------------------
make test-go TEST_PKG_DIRS="internal/..."
make[1]: Entering directory '/home/t25kim/work/edge-home-orchestration-go'
gocov test -v ./internal/... > coverage.out
/bin/sh: 1: gocov: not found

t25kim@t25kim:~/work/edge-home-orchestration-go$ go get github.com/axw/gocov/gocov
go get: added github.com/axw/gocov v1.0.0
t25kim@t25kim:~/work/edge-home-orchestration-go$ make test
.
.
github.com/lf-edge/edge-home-orchestration-go/internal/restinterface/tls/tls.go  SetHandler                              0.00% (0/1)
github.com/lf-edge/edge-home-orchestration-go/internal/restinterface/tls         -------------------------------------   94.44% (17/18)

Total Coverage: 73.75% (3071/4164)
gocov-html coverage.out > coverage.html
/bin/sh: 1: gocov-html: not found