Fix NullReferenceException in tests

[martincostello]

If multiple unit tests are running in parallel, it is possible for the Add() operation on the dictionary to throw a NullReferenceException internally and cause a test to fail.

Fix this by using ConcurrentDictionary instead.

Originally part of #131.

[StanleyGoldman]

How are you able to reproduce this NullReferenceException

I'm using NCrunch's churn mode to try to reproduce this test running ExpressionRewriterTests endlessly.
I can't seem to get the failure.

[martincostello]

I just use the Visual Studio Test Explorer.

I right click on the .UnitTest node and click Run Selected Tests.

This is from the Update-Test-Target-Framework branch as I don't have .NET Core 1.x installed on my machine so the tests don't run otherwise (it's what motivated me to make the change in #140).

[StanleyGoldman]

Finally reproduced

[StanleyGoldman]

So a sure fire way for me to reproduce this is to run all three test namespaces once with the Test Explorer.
Then to continually use the "Repeat Last Run" function until i get the failure.

[StanleyGoldman]

That happened for me on the current master

[martincostello]

Must be an implicit dependency between the suites doing stuff with the trees, plus the default xunit level of parallelism is to run test assemblies against each other in parallel.

[StanleyGoldman]

So now i'm testing this pull request using the same means as above, and I'm able to push it to a different race condition. @martincostello wanna see if you can repro/fix this?

[martincostello]

Interesting - I didn't get that one. I'll take a look in a bit and see if I can make that case behave too.

[martincostello]

@StanleyGoldman Want to pull the latest commit and give that a try?

[StanleyGoldman]

I'm unable to get it to crash with these changes. Good 💩.

[grokys]

LGTM! Thanks @martincostello!