Make patterns in `testSubsetGrep.properties` platform-agnostic

[AndreasArvidsson]

Today when you are on windows you have to use \\ in testSubsetGrep.properties. I would prefer to use / on all platforms.

@pokey suggestion that we might normalize the names here:

cursorless/packages/common/src/testUtil/getFixturePaths.ts

Line 31 in 63ea8e9

name: path.relative(relativeDir, p.split(".")[0]),