Need unit tests of the core library

It would be nice if we could write unit tests for just the core library so that we have an easy way to investigate problems in the core library without relying on the full integration tests that run the delta generator from within MSBuild.

One complication is that the library is written around using an `MSBuildWorkspace` for the baseline project. So even "unit" tests still need a pre-built baseline library as a starting point. But the library shouldn't mutate the on-disk baseline, so we can probably get away with a single baseline project that all the tests could use as a starting point.

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At the moment the only way to really debug the library is by building the `hotreload-delta-gen` console tool and the example in src\hotreload-delta-gen\example and then running the tool by hand on the baseline example from within a C# debugger. But this is annoying because you have to specify the paths for the msbuild project, the delta script, etc by hand in the debugging configuration. It would be much nicer if we could debug unit tests.