test: migrate slow tests to use vitest

[erickzhao]

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This PR moves Electron Forge's slow test suite to use Vitest as a follow-up to #3797. A few notes:

Test migration

Migrating these tests was a lot more straightforward because they run raw commands via child process instead of asserting against mocks. These tests didn't use sinon/proxyquire at all, so I just had to change the Mocha/Chai test harness and assertion calls over to Vitest.

Fixture migration

I moved all files over to /spec/ including test fixtures, so the actual code diff is smaller than it seems.

package.json cleanup

• I also removed all test npm scripts from each individual package's package.json because we run a single instance of vitest from the root of the package rather than using lerna to execute each package's individual scripts.
• chai, mocha, and associated packages are gone!

Removal of unnecessary test utils

I removed expectProjectPathExists and expectProjectPathNotExists from @electron-forge/test-utils since these helper functions were essentially calling fs.existsSync with extra steps.

Workspace organization

The fast and slow suites are now assigned as Vitest workspace projects based on the test file names.

• vitest.config.mts is the base file.
• vitest.workspace.mts contains separate configs for slow and fast tests (mostly just increased test timeouts for the slow test suite).

Note that I had to turn off file parallelism for the time being because there were issues with running multiple test files at once (potentially around FS collisions?). Once this lands, I'll open an issue to track the problem.

[dsanders11]

Another awesome PR! 🎉

I think we could also drop the cross-env dependency in this PR? Looks like it was only used with the Mocha test script commands. Should be able to remove it from package.json and packages/api/core/package.json.

[dsanders11]

Can remove this dependency.

[dsanders11]

Suggested change

	"recommendations": ["dbaeumer.vscode-eslint", "esbenp.prettier-vscode"]
	"recommendations": ["dbaeumer.vscode-eslint", "esbenp.prettier-vscode", "vitest.explorer"]

The Vitest extension works well and is maintained by the Vitest team, let's swap it in here?