test Flow with babel-flow parser

[brodybits]

I noticed that PR #8508 removed some testing of Flow with the babel-flow parser.

This PR adds testing with babel-flow to almost all tests in tests/flow-repo and tests/flow. The one exception is tests/flow-repo/generics.

The test from tests/flow-repo/union_new/issue-815.js is moved into its own subdirectory since it is the only one that gets a different snapshot for babel-flow vs flow parser.

• I’ve added tests to confirm my change works.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[fisker]

The test from tests/flow-repo/union_new/issue-815.js is moved into its own subdirector

We can't move files in flow-repo, it's from other project, you can copy it in to flow and run test from there.

[fisker]

superseded by #14245