Use `memfs` to test filesystem operations in unit tests

[t1m0thyj]

Recently we had an issue (fixed in #2188) where we were writing new contents to a JSON file that were shorter than the old contents without truncating the file, which resulted in invalid JSON.

With our current unit tests that mock fs methods using Jest, we don't have a way to detect this kind of bug. If we added tests that use the memfs package to mock out the file system in memory, we could.

Places where these kinds of tests could be useful:

• Places where call fs.writeFileSync: https://github.com/zowe/vscode-extension-for-zowe/blob/15fc59a54a57b7e5012e6c5a56af5f6fd529b160/packages/zowe-explorer/src/utils/ProfilesUtils.ts#L365
• Any places where we call mainframe APIs to download a file to disk

Resources

• https://www.npmjs.com/package/memfs
• https://medium.com/nerd-for-tech/testing-in-node-js-easy-way-to-mock-filesystem-883b9f822ea4

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