add and update zip util tests with ZipReadCloser coverage

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

internal/file/zip_read_closer_test.go

@@ -0,0 +1,47 @@
+package file
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFindArchiveStartOffset(t *testing.T) {
+	tests := []struct {
+		name        string
+		archivePrep func(tb testing.TB) string
+		expected    uint64
+	}{
+		{
+			name:        "standard, non-nested zip",
+			archivePrep: prepZipSourceFixture,
+			expected:    0,
+		},
+		{
+			name:        "zip with prepended bytes",
+			archivePrep: prepZipSourceFixtureWithPrependedBytes(t, "junk at the beginning of the file..."),
+			expected:    36,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			archivePath := test.archivePrep(t)
+			f, err := os.Open(archivePath)
+			if err != nil {
+				t.Fatalf("could not open archive %q: %+v", archivePath, err)
+			}
+			fi, err := os.Stat(f.Name())
+			if err != nil {
+				t.Fatalf("unable to stat archive: %+v", err)
+			}
+
+			actual, err := findArchiveStartOffset(f, fi.Size())
+			if err != nil {
+				t.Fatalf("unable to find offset: %+v", err)
+			}
+			assert.Equal(t, test.expected, actual)
+		})
+	}
+}