tstest/integration/vms: regex-match distros using a flag (#2021)

If you set `-distro-regex` to match a subset of distros, only those
distros will be tested. Ex:

    $ go test -run-vm-tests -distro-regex='opensuse'

Signed-off-by: Christine Dodrill <xe@tailscale.com>

tstest/integration/vms/regex_flag_test.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vms
+
+import (
+	"flag"
+	"testing"
+)
+
+func TestRegexFlag(t *testing.T) {
+	var v regexValue
+	fs := flag.NewFlagSet(t.Name(), flag.PanicOnError)
+	fs.Var(&v, "regex", "regex to parse")
+
+	const want = `.*`
+	fs.Parse([]string{"-regex", want})
+	if v.Unwrap().String() != want {
+		t.Fatalf("got wrong regex: %q, wanted: %q", v.Unwrap().String(), want)
+	}
+}