tstest/integration/vms: codegen for top level tests (#2441)

This moves the distribution definitions into a maintainable hujson file
instead of just existing as constants in `distros.go`. Comments are
maintained from the inline definitions.

This uses jennifer[1] for hygenic source tree creation. This allows us
to generate a unique top-level test for each VM run. This should
hopefully help make the output of `go test` easier to read.

This also separates each test out into its own top-level test so that we
can better track the time that each distro takes. I really wish there
was a way to have the `test_codegen.go` file _always_ run as a part of
the compile process instead of having to rely on people remembering to
run `go generate`, but I am limited by my tools.

This will let us remove the `-distro-regex` flag and use `go test -run`
to pick which distros are run.

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

tstest/integration/vms/distros_test.go

@@ -0,0 +1,17 @@
+// 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.
+
+// +build linux
+
+package vms
+
+import (
+	"testing"
+)
+
+func TestDistrosGotLoaded(t *testing.T) {
+	if len(Distros) == 0 {
+		t.Fatal("no distros were loaded")
+	}
+}