test: use a fixed length name for temp directories (#2544)

Most of the time `os.MkdirTemp` creates directories with the same number
of characters, but its not required to and will occasionally has one
less character than what is common, which can result in the table output
being slightly different.

This is compounded by us replacing the temp directory with a fixed
string `<tempdir>` marker, as it ends up looking like the table marks
are just randomly changing for no reason 🤦

Author: G-Rath

internal/testutility/utility.go

@@ -3,7 +3,9 @@ package testutility
 import (
 	"flag"
 	"fmt"
+	"math/rand"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"testing"
@@ -105,14 +107,23 @@ func ValueIfOnWindows(win, or string) string {
 	return or
 }
 
+func fixedLengthTempDir(parent string) (string, error) {
+	n := rand.Int63n(1_000_000_000_000) //nolint:gosec // 10^12
+	suffix := fmt.Sprintf("%0*d", 12, n)
+
+	name := "osv-scanner-test-" + suffix
+	path := filepath.Join(parent, name)
+
+	return path, os.Mkdir(path, 0o700)
+}
+
 // CreateTestDir makes a temporary directory for use in testing that involves
 // writing and reading files from disk, which is automatically cleaned up
 // when testing finishes
 func CreateTestDir(t *testing.T) string {
 	t.Helper()
 
-	//nolint:usetesting // we need to customize the directory name to replace in snapshots
-	p, err := os.MkdirTemp("", "osv-scanner-test-*")
+	p, err := fixedLengthTempDir(os.TempDir())
 	if err != nil {
 		t.Fatalf("could not create test directory: %v", err)
 	}