feat: make load, setFlows async/await (enable vitest)

[AllanOricil]

This is a continuation of #81 to enable vitest as the test runner

Repro Steps

1. clone https://github.com/AllanOricil/node-red-node-test-helper
2. checkout branch vitest-setup-2
3. npm install
4. npm link
5. clone https://github.com/AllanOricil/nrg-template
6. checkout branch vitest
7. npm run install
8. npm link node-red-node-test-helper
9. npm run test
10. verify tests pass as displayed below

[AllanOricil]

BEFORE

In the image below it is possible to see that I'm using the package without any of my changes

AFTER

All tests from node-red are still passing with these changes. In the image below it is possible to see that I linked my changes in the node-red repo before running tests with it.

Conclusion

These changes do not affect the current behavior of the test lib

[knolleary]

Looks good - thanks!