Vim: detect JS test files containing JSX

We were detecting JS test files and JS files with JSX, but not JS test
files containing JSX.

I presume there's some ordering involved here; ftdetect/jasmine.vim
probably runs first and sets the file type to "javascript.jasmine" and
then ftdetect/jsx.vim comes along and clobbers the file type to
"javascript.jsx". That's my guess anyway.

Another alternative to is to look at seeing if these can add to the
filetype instead of overwriting it; will check that out separately.