Move the send transactions test to the main branch

[teor2345]

Motivation

The send transactions test is very slow and copies a lot of data.

We want to fix this test in #5015. But until then, we want to move it to the main branch.

If we move it to the main branch, we'll have a better chance of catching any changes that break this RPC.

Designs

Move the workflow to main like we did in PR #5393.

Rejected Alternatives

If we turn this test off, we need to either:

• test it manually before every release, or
• accept the risk that it might be broken in a release.

We're not planning on making any changes to this RPC, but we are working on some related RPC and mempool code that it calls.

Move the job definition into a separate workflow file, and turn off that workflow in GitHub Actions.

[teor2345]

@mpguerra this is the slow test we talked about this morning

[mpguerra]

Let's move to main for now and see how it goes

[mpguerra]

Hey team! Please add your planning poker estimate with Zenhub @arya2 @conradoplg @oxarbitrage @teor2345 @upbqdn