Update readme with instructions to run individual groups of tests and prerequisites for deploy test run

[kvenkatrajan]

@jeo02 with the prior task we have groupings of tests. lets add it to the readme instructions for how to run deploy tests.

npm test -- azure-deploy/integration.test.ts --testNamePattern="static-web-apps-deploy"

for example requires
azd login
az login and
default subscription before starting the run

[kvenkatrajan]

#796