Tests: extract the duplicated console-capture and output-folder scaffolding

[SkowronskiAndrew]

The test fixtures in UnityDataTool.Tests each re-implement the same two pieces of scaffolding.

Running a command and capturing its output. Every fixture that needs to assert on what a command
printed redirects Console.Out / Console.Error into a StringWriter, calls Program.Main, and
restores the originals in a finally. There are three named variants of this —
AnalyzeOverwriteTests.RunAnalyze (stderr only), AnalyzeExitCodeTests.RunAnalyze (both streams),
FindRefsTests.RunFindRefsOn — plus a large number of copies written inline inside individual test
methods, for example the two no-TypeTree tests in UnityDataToolPlayerDataTests.

Output folder setup and teardown. OneTimeSetup creates a per-fixture folder under the test
directory and SetCurrentDirectorys into it; Teardown calls SqliteConnection.ClearAllPools() and
deletes the folder's files and subdirectories. This block is duplicated nearly verbatim across
roughly 15 fixtures.

Neither is causing bugs, but it means a change to how tests run a command or clean up has to be made
in many places, and each new fixture starts by copying 40 lines.

Suggested shape:

• A shared helper that runs a command and returns (exitCode, stdout, stderr) — or combined output,
  which is what most assertions want. SQLTestHelper is the project's existing shared helper but is
  SQL-specific, so a small separate helper class is probably a better home.
• A base fixture (or a helper method) for the output-folder creation and teardown.

Then migrate the fixtures over, which can be done incrementally.

Raised during the review of the fix for #115, which added one more copy of both patterns.