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/gen/test_codegen.go

@@ -0,0 +1,55 @@
+// 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 ignore
+
+package main
+
+import (
+	_ "embed"
+	"fmt"
+	"log"
+	"os"
+	"time"
+
+	"github.com/dave/jennifer/jen"
+	"github.com/iancoleman/strcase"
+	"tailscale.com/tstest/integration/vms"
+)
+
+func main() {
+	f := jen.NewFile("vms")
+	f.Comment("Code generated by tstest/integration/vms/gen/test_codegen.go DO NOT EDIT.")
+
+	ptr := jen.Op("*")
+
+	for i, d := range vms.Distros {
+		f.Func().
+			Id("TestRun" + strcase.ToCamel(d.Name)).
+			Params(jen.Id("t").Add(ptr).Qual("testing", "T")).
+			BlockFunc(func(g *jen.Group) {
+				g.Id("t").Dot("Parallel").Call()
+				g.Id("setupTests").Call(jen.Id("t"))
+				g.Id("testOneDistribution").Call(jen.Id("t"), jen.Lit(i), jen.Id("Distros").Index(jen.Lit(i)))
+			})
+	}
+
+	os.Remove("top_level_test.go")
+	fout, err := os.Create("top_level_test.go")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer fout.Close()
+
+	fmt.Fprintf(fout, "// Copyright (c) %d Tailscale Inc & AUTHORS All rights reserved.\n", time.Now().Year())
+	fout.WriteString("// Use of this source code is governed by a BSD-style\n")
+	fout.WriteString("// license that can be found in the LICENSE file.\n")
+	fout.WriteString("\n")
+	fout.WriteString("// +build linux\n\n")
+
+	err = f.Render(fout)
+	if err != nil {
+		log.Fatal(err)
+	}
+}