Rewrite test data logic to make it faster

It turns out we have a few tests with thousands of test data files,
which this logic does not handle very well. Just switching from `x :=
$(x) ...` to `x += ...` accomplishes the majority of these gains, but I
rewrote the logic to be less function-call+eval based to hopefully make
it easier to understand.

For an aosp-master/aosp_coral-eng build, this brings the time spent in
this logic from 7.6s to 0.3s (total kati runtime of 54s to 46s). Kati's
peak memory use (maxrss) also shrunk from 6.1GB to 3.8GB.

Bug: 158488548
Test: build-aosp_coral.ninja is the same before/after
Change-Id: Ia2ec8b29b5c1dd12fa7ea10b4f80fe802c692725