chore: bring back testcontainers tests

[mdelapenya]

Hi from the Testcontainers community! 👋

PR Description

This PR bumps the version of Testcontainers for Go, to the latest released: v0.31.0, unskipping all the tests from the code (skipped in https://github.com/grafana/agent/pull/4413).

At the same time, given the project uses testcontainers-go for Vault, I'm including the vault module, in case it's useful.

Which issue(s) this PR fixes

• Relates https://github.com/grafana/agent/pull/5944 https://github.com/grafana/agent/pull/5944

Notes to the Reviewer

I've run the Vault tests locally, even the dev port is using the default in the module (8200/tcp, here it was 80/tcp).

I'm currently checking the existing backlog, and #6670 could conflict with this one, as it keeps some t.Skip but not for testcontainers reasons. See https://github.com/grafana/agent/pull/6670/files#diff-1f0f435cab418ca5c8a2e2d6411411bcaf85c131298e21082678de8a33f1adc1R143

PR Checklist

• [ ] CHANGELOG.md updated
• [ ] Documentation added
• [x] Tests updated
• [ ] Config converters updated

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