Allow passing a list of glob patterns in testFiles config option

[bahmutov]

Cypress v3.4.1

There is a configuration option testFiles in https://on.cypress.io/configuration that controls which spec files are found.
Let's say I have the following files

cypress/
	integration/
		foo.js
		bar.js
		baz.js

If I want to only have "foo.js" file I can do the following

{
  "testFiles": "foo.js"
}

But how do I pick "foo.js" AND "bar.js"?

I would be very nice to allow testFiles to be a single string or a list of strings and match each filename against a list.

[bahmutov]

Example repo in https://github.com/cypress-io/cypress-test-tiny/tree/test-files

[bahmutov]

This would be super useful for JS specs + Markdown fiddles in cypress-io/cypress-fiddle#21 where I need to select a pattern of spec filenames AND all markdown files

[bahmutov]

We could follow the example of ignoreTestFiles: v.isStringOrArrayOfStrings (config.coffee) and update testFiles to be similar

[bahmutov]

Since we use glob under the hood (should note this in our configuration docs), then to select two files one should use

{
  "testFiles": "{foo,bar}.js"
}

and you can see the spec finding messages by running Cypress with DEBUG=cypress:server:specs environment variable

[bahmutov]

Note that there should be no spaces in the pattern. To find all files with extension .md AND ending with spec.js one needs to do

{
  "testFiles": "{*.md,*spec.js}",
}