CI: Add workflow_dispatch windows test suite CI

[ptheywood]

Currently, the only time the test suite is built on windows (with warnings as errors enabled) is when doing a full draft release CI run, or making a draft release.
It is not enabled for regular per-push ci builds due to the time required.

As MSVC emits different warnings in some cases to gcc, it would be good to be able to run this (alone) manually.

The simplest option, with the lest duplication of workflow, will be to modify the condition of the build test step, so that if the regular windows ci was triggered by a workflow_dispatch to also build the test suite.

Alternatively a separate workflow could be created, to avoid running multiple CI jobs when one would be sufficient.

Or we could enable it on every build, but this would greatly slow down CI due to how long this takes to build when 2 cores/threads are available.