cmd/go: test that the reserved paths "example" and "test" are not used
cmd/go: test that the reserved paths "example" and "test" are not used

Proposal #37641 reserved the paths "example" and "test" for end users,
so no path beginning with either of those elements may be added to the
standard library.

(We are unlikely to add them by accident, but this test codifies the
policy accepted in the proposal.)

Fixes #37641

Change-Id: I999064180336c923d4918a1176c371b8f5666f8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/359576
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go