Allow specifying individual specific tests to run with `flow test`

[jribbink]

Issue To Be Solved

It would be useful to have the ability to specify an individual test to run when using the flow test command.

I.e. for

access(all) fun testFoobar() {
...
}

access(all) fun testBarFoo() {
...
}

It would be nice to be able to just run one of these tests instead of all of them.

(Optional): Suggest A Solution

Add a flag like --run to flow test. From here a user could specify a regular expression to match against test functions & filter the tests which they wish to run.

(Optional): Context

This is important for any IDE/external tooling supporting of the Cadence test framework. Specifically, to integrate with the VSCode Testing API, it's important to have the ability to target individual test runs beyond a file/folder level.