Add a parameter to go_test to support a custom TestMain template

[shirchen]

We want to opt-in our monorepo into goroutine leak detection and we plan on using uber-go/leak. One option for us is to change template to something like

+.    {{if not .TestMain}}
+	 "go.uber.org/goleak"
+.    {{end}}
 	{{if not .TestMain}}
-	res := m.Run()
+       goleak.VerifyTestMain(m)
 	{{else}}
 	{{.TestMain}}(m)
 	{{/* See golang.org/issue/34129 and golang.org/cl/219639 */}}
 	res := int(reflect.ValueOf(m).Elem().FieldByName("exitCode").Int())
-	{{end}}
 	os.Exit(res)

But then we need to introduce goleak as a dependency to rules_go which may or may not be acceptable. One way we can think of making this optional is to allow us to pass in a custom template that we will have and will give us flexibility. A potential parameter could be

        "test_main_template": attr.label(
            default = "//go/tools/builders/templates:test_main",
            allow_file = True,
        ),

Note, we still need a way to add this dependency (deps = ["@org_uber_go_goleak//:go_default_library"]). One option is to make _testmain_additional_deps public.