x/tools/gopls: TestSwitchFromGOPATHToModuleMode failing in longtest builders

[findleyr]

https://go.dev/cl/459785 inadvertently broke the x/tools longtest builders (the failing test no longer makes sense in one of the longtest modes).

Fix incoming.

[gopherbot]

Change https://go.dev/cl/459792 mentions this issue: gopls/internal/regtest/watch: don't run TestSwitchFromGOPATHToModuleMode