Assuming that your test framework supports it, what I would suggest is that instead of implementing the tests you want to braindump, instead write descriptive pending tests that you will later implement. For example, if your API should do foo and bar but not biz, just add the following code (this example is in rspec) for your test suite, then attack them one by one. You get your thoughts down quickly and can address all your issues one by one. When all the tests pass you will know when you have addressed all your issues you had during your braindump.

describe "Your API" do

  it "should foo" do
    pending "braindump from 4/2"
  end

  it "should bar" do
    pending "braindump from 4/2"
  end

  it "should not biz" do
    pending "braindump from 4/2"
  end

end