clang/test/CodeCompletion: Simplify/fix some `touch` commands

These were inconsistent (one `touch` line listed all the files in a
single `touch` invocation, but the other listed them as separate
commands with `&&`) and incorrect (one `&&` was missing, causing `touch`
to try to touch a file called `touch` in the cwd, which might not be
accessible/relevant to test execution)

So make them consistent and simpler by using the "list all the files on
a line in a single `touch` invocation" reducing the visual
noise/clutter/etc.