## What's the problem this PR addresses?

The integration tests using custom certificates have been failing for
quite a while on Windows with the error `RequestError: unsuitable
certificate purpose`.

## How did you fix it?

This error was caused by the test CA certificate used; it was not
configured to be used as a CA when it was generated.

The test script that generates the CA certificate has been updated to
generate it correctly. I used the same configuration options used by the
test suite of the `pem` library that we are using for certificate
generation:
https://github.com/Dexus/pem/blob/3d71b87cec08a565ce5e73e26dd18f53b9e1fdea/test/pem.spec.js#L856

The problem does not seem to be specific to Windows; I was able to
reproduce it locally on macOS as well. I don't yet know why it's passing
on CI for Ubuntu and macOS, but not for Windows. Perhaps some difference
in the images, like the OpenSSL version (they should all be using
OpenSSL v1.1.1, but I've seen reports of Windows having v3 installed as
well).

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