test: add test coverage script (#1014)

This PR adds a new test coverage script and corresponding Makefile
target to improve visibility into code coverage metrics. The script
provides a flexible way to generate coverage reports with optional HTML
output.

Easy to diff:

````
% diff coverage.before coverage.after 
14c14
< ok  	oras.land/oras/cmd/oras/internal/display/metadata/text	(cached)	coverage: 31.5% of statements
---
> ok  	oras.land/oras/cmd/oras/internal/display/metadata/text	0.270s	coverage: 41.4% of statements
155,157c155,157
< oras.land/oras/cmd/oras/internal/display/metadata/text/attach.go:36:			NewAttachHandler		0.0%
< oras.land/oras/cmd/oras/internal/display/metadata/text/attach.go:43:			OnAttached			0.0%
< oras.land/oras/cmd/oras/internal/display/metadata/text/attach.go:56:			Render				0.0%
---
> oras.land/oras/cmd/oras/internal/display/metadata/text/attach.go:36:			NewAttachHandler		100.0%
> oras.land/oras/cmd/oras/internal/display/metadata/text/attach.go:43:			OnAttached			100.0%
> oras.land/oras/cmd/oras/internal/display/metadata/text/attach.go:56:			Render				100.0%
663c663
< total:											(statements)		34.8%
---
> total:											(statements)		35.1%
````

---------

Signed-off-by: Terry Howe <terrylhowe@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: TerryHowe

Makefile

@@ -15,6 +15,12 @@
 test: vendor check-encoding
 	go test -race -v -coverprofile=coverage.txt -covermode=atomic ./...
 
+.PHONY: test-coverage
+test-coverage:  ## look at code coverage
+	@echo
+	@echo "==> Running unit tests with coverage: $(PKG) <=="
+	@ ./scripts/coverage.sh $(PKG)
+
 .PHONY: covhtml
 covhtml:
 	open .cover/coverage.html

scripts/coverage.sh

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+
+# Copyright The ORAS Authors.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -euo pipefail
+
+covermode=${COVERMODE:-atomic}
+coverdir=$(mktemp -d /tmp/coverage.XXXXXXXXXX)
+trap 'rm -rf "${coverdir}"' EXIT
+profile="${coverdir}/cover.out"
+html=false
+target="./..." # by default the whole repository is tested
+for arg in "$@"; do
+  case "${arg}" in
+    --html)
+      html=true
+      ;;
+    *)
+      target="${arg}"
+      ;;
+  esac
+done
+
+generate_cover_data() {
+  for d in $(go list "$target"); do
+    (
+      local output="${coverdir}/${d//\//-}.cover"
+      go test -coverprofile="${output}" -covermode="$covermode" "$d"
+    )
+  done
+
+  echo "mode: $covermode" >"$profile"
+  grep -h -v "^mode:" "$coverdir"/*.cover >>"$profile"
+}
+
+generate_cover_data
+go tool cover -func "${profile}"
+
+if [ "${html}" = "true" ] ; then
+    go tool cover -html "${profile}"
+fi
+