txtar: add UTF-8 test case in TestParse

Adds a new test case for the txtar package to verify that:

Parse correctly handles UTF-8 characters in file contents.
Leading comment lines are preserved properly.
This improves test coverage for non-ASCII content and ensures
comment parsing works as expected.

Change-Id: I3eff5e09f6c3c588ca817c36178c9a7bafaa4ea7
GitHub-Last-Rev: 920f684
GitHub-Pull-Request: #600
Reviewed-on: https://go-review.googlesource.com/c/tools/+/712980
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

Author: saurabhgame

txtar/archive_test.go

@@ -52,6 +52,22 @@ some content
 				Files:   []File{{"file", []byte("data\r\n")}},
 			},
 		},
+		{
+			name: "utf8 and comment",
+			text: `# This is a test comment
+-- hello.txt --
+Hello
+-- unicode.txt --
+Go语言
+`,
+			parsed: &Archive{
+				Comment: []byte("# This is a test comment\n"),
+				Files: []File{
+					{"hello.txt", []byte("Hello\n")},
+					{"unicode.txt", []byte("Go语言\n")},
+				},
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {