Update testing documentation (#5504)

### Description
Fixes a broken link in our testing documentation.

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### How has this been tested?
[Can be tested here

](https://github.com/dafny-lang/dafny/blob/5c292792e4da0d59cadf6bbef034d2577ef6f4ea/Source/IntegrationTests/README.md)
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<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/IntegrationTests/README.md

@@ -1,4 +1,4 @@
 # IntegrationTests
 
-This .NET test project executes all of the LIT integration tests under the [`Test`](../../Test/) directory
+This .NET test project executes all of the LIT integration tests under the [`LitTest`](TestFiles/LitTests/LitTest) directory
 using the [xUnit-based LIT interpreter](../XUnitExtensions/).