Simple README for the Unit Tests.

- How to run.
- Summary of suites.
- WSL2 time bug.

src/FileCache.UnitTests/README.md

@@ -0,0 +1,49 @@
+Unit-Testing FileCache
+------------------------
+
+This will tell dotnet to build FileCache and run all of the tests.
+
+``` shell
+cd /path/to/FileCache/src/FileCache.UnitTests
+dotnet test FileCache.UnitTests.csproj
+```
+
+Unit-Test Suites
+----------------
+
+There are two test suites:
+
+1. FileCacheTest.cs
+   - These tests use the BasicFileCacheManager when running various unit-tests.
+
+1. HashedFileCache.cs
+   - These tests are similar, but use the HashedFileCacheManager.
+
+
+WSL2 Warning!
+----------------
+
+Sometimes a WSL operating system's time can get out-of-sync with the actual time (see [this](https://github.com/microsoft/WSL/issues/4114)). If it's out of sync enough, some tests can fail if Windows is ultimately in charge of file stats like Last Accessed Time. This happens if your FileCache repo is on a Windows drive (i.e. something under /mnt/c, /mnt/d, etc).
+
+For example, FileCache.FlushRegionTest will fail with this error when your WSL'd Linux OS is 1 minute in the past:
+
+```
+Failed FlushRegionTest [1 s]
+Error Message:
+ Expected result2 to be <null>, but found "Value2".
+Stack Trace:
+   at FluentAssertions.Execution.LateBoundTestFramework.Throw(String message)
+ at FluentAssertions.Execution.TestFrameworkProvider.Throw(String message)
+ at FluentAssertions.Execution.DefaultAssertionStrategy.HandleFailure(String message)
+ at FluentAssertions.Execution.AssertionScope.FailWith(Func`1 failReasonFunc)
+ at FluentAssertions.Execution.AssertionScope.FailWith(Func`1 failReasonFunc)
+ at FluentAssertions.Execution.AssertionScope.FailWith(String message, Object[] args)
+ at FluentAssertions.Primitives.ReferenceTypeAssertions`2.BeNull(String because, Object[] becauseArgs)
+ at FC.UnitTests.FileCacheTest.FlushRegionTest() in /mnt/path/to/FileCache/src/FileCache.UnitTests/FileCacheTest.cs:line 266
+```
+
+To fix, you can try:
+    `sudo hwclock -s`
+
+If that doesn't work, you'll probably have to shutdown and restart the VMs:
+    `wsl --shutdown`