Add MakeMemMapFs to testutils package

While working on another PR I was looking for a way to quickly
initialize a test Fs. I found in the codebase occurences of a
makeMemMapFs function, which was already duplicated twice in the
codebase.

Instead of adding a third copy, I thought I would instead make testutil
out of it. This is exactly what this commit does.

lib/testutils/fs.go

@@ -0,0 +1,28 @@
+package testutils
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.k6.io/k6/lib/fsext"
+)
+
+// MakeMemMapFs creates a new in-memory filesystem with the given files.
+//
+// It is particularly useful for testing code that interacts with the
+// filesystem, as it  allows to create a filesystem with a known state
+// without having to create temporary directories and files on disk.
+//
+// The keys of the withFiles map are the paths of the files to create, and the
+// values are the contents of the files. The files are created with 644 mode.
+//
+// The filesystem is returned as a [fsext.Fs].
+func MakeMemMapFs(t *testing.T, withFiles map[string][]byte) fsext.Fs {
+	fs := fsext.NewMemMapFs()
+
+	for path, data := range withFiles {
+		require.NoError(t, fsext.WriteFile(fs, path, data, 0o644))
+	}
+
+	return fs
+}