tests: Improve test-plugin-output-streaming.sh diagnostics and robust

Author: euanh

scripts/test-plugin-output-streaming.sh

@@ -1,8 +1,21 @@
 #!/usr/bin/env bash
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the SwiftContainerPlugin open source project
+##
+## Copyright (c) 2025 Apple Inc. and the SwiftContainerPlugin project authors
+## Licensed under Apache License v2.0
+##
+## See LICENSE.txt for license information
+## See CONTRIBUTORS.txt for the list of SwiftContainerPlugin project authors
+##
+## SPDX-License-Identifier: Apache-2.0
+##
+##===----------------------------------------------------------------------===##
 
 # Test that error output streamed from containertool is printed correctly by the plugin. 
 
-set -exo pipefail
+set -euo pipefail
 
 log() { printf -- "** %s\n" "$*" >&2; }
 error() { printf -- "** ERROR: %s\n" "$*" >&2; }
@@ -16,23 +29,27 @@ cleanup() {
 }
 trap cleanup EXIT
 
+# Create a test project which depends on this checkout of the plugin repository
+REPO_ROOT=$(git rev-parse --show-toplevel)
 swift package --package-path "$PKGPATH" init --type executable --name hello
 cat >> "$PKGPATH/Package.swift" <<EOF
 package.dependencies += [
-    .package(path: "$PWD"),
+    .package(path: "$REPO_ROOT"),
 ]
 EOF
+cat "$PKGPATH/Package.swift"
 
 # Run the plugin, forgetting a mandatory argument.   Verify that the output is not corrupted.
 # The `swift package` command will return a nonzero exit code.   This is expected, so disable pipefail.
 set +o pipefail
 swift package --package-path "$PKGPATH" --allow-network-connections all build-container-image 2>&1 | tee "$PKGPATH/output"
 set -o pipefail
 
+# This checks that the output lines are not broken, but not that they appear in the correct order
 grep -F -x -e "error: Missing expected argument '--repository <repository>'" \
            -e "error: Help:  --repository <repository>  Repository path" \
            -e "error: Usage: containertool [<options>] --repository <repository> <executable>" \
            -e "error:   See 'containertool --help' for more information." "$PKGPATH/output"
 
-echo Plugin error output: PASSED
+log Plugin error output: PASSED