Test Refactor: Investigate enabling parallel tests

### Description

We currently have parallel tests disabled.

https://github.com/Azure/azure-functions-core-tools/blob/37b826f143b99cd62308b88320e71aa806f6197f/test/Azure.Functions.Cli.Tests/xunit.runner.json#L2

Could we investigate what can be done so that it can be enabled? Could we run multiple func.exe process at same time, with unique port numbers?

Start at test/Azure.Functions.Cli.Tests/xunit.runner.json line 2, where parallel tests are disabled, and inspect how the tests launch func.exe processes. Investigate whether concurrent processes can use unique port numbers without interfering. Done means parallel testing can be enabled safely, or the blocking constraints are documented.

Written by the indexing model from the issue text.