gopls/internal/regtest/watch: don't run TestSwitchFromGOPATHToModuleMode

in experimental mode

This test no longer works with 'experimentalWorkspaceModule' set. That
option is being removed, but in the meantime only run the test in
default mode, to unbreak the longtest builders.

Fixes golang/go#57521

Change-Id: I95083f577f93116c342787eabf75e36cd05df9f3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/459792
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

gopls/internal/regtest/watch/watch_test.go

@@ -589,6 +589,7 @@ func main() {
 `
 	WithOptions(
 		InGOPATH(),
+		Modes(Default), // golang/go#57521: this test is temporarily failing in 'experimental' mode
 		EnvVars{"GO111MODULE": "auto"},
 	).Run(t, files, func(t *testing.T, env *Env) {
 		env.OpenFile("main.go")