fix(test): Wait for Vault server in watch tests

[thevilledev]

Fix flaky tests by polling Vault's /v1/sys/health endpoint. This ensures the server is ready before tests attempt to connect, preventing a race condition during Vault dev server startup.

Example of a failed run: https://github.com/hashicorp/consul-template/actions/runs/14620430375/job/41018777729?pr=2052

Breakdown of the issue in watch/watch_test.go:

• main calls newTestVault() to start a Vault dev server using exec.Command("vault", "server", "-dev", ...) and stores the command in testVault.
• Immediately after cmd.Start(), main creates Vault clients and calls vaultTokenSetup(clients).
• vaultTokenSetup then attempts to communicate with the Vault API through vc.Sys().EnableAuthWithOptions(...), which fails with the connection refused error as the process is not in listening state yet.

Seems to affect the Enterprise tests more than the others.

You can simulate the test error locally by making the test launch an intermediate script which sleeps indefinitely.

[thevilledev]

I was able to dig out the following from Vault (Enterprise) stdout:

Error parsing listener configuration.
Error initializing listener of type tcp: listen tcp 127.0.0.1:8200: bind: address already in use

Looks like it needs to use ephemeral ports like the other tests. Will mark this as a draft for now.

[thevilledev]

Now watch package tests pass. However, all Consul Enterprise related tests still fail.

failed to start consul server: api unavailable
FAIL	github.com/hashicorp/consul-template/dependency	2.154s

There's very little to debug due to consul output being omitted. My only hunch is that maybe the Consul Enterprise license in CONSUL_LICENSE env variable (passed from a Github secret) has been expired?

Maybe someone from HashiCorp could verify.