docs: to README.md add tests troubleshooting (run in series)

[riverKanies]

Description

This PR addresses https://github.com/bitcoindevkit/bdk_wallet/issues/43 Shows how to force tests to run in series (rather than parallel) in case of issues running tests that use bitcoind or electrsd

Notes to the reviewers

Can also enforce running in series like https://github.com/bitcoindevkit/bdk/pull/1728 I figured it wouldn't hurt to add this to the README.md anyway so made a PR for both

Bugfixes:

• [x] I'm linking the issue being fixed by this PR