Add testing for feedstocks with preprocessor functions

[cisaacstern]

Preprocessor functions in feedstocks can cause serialization issues, e.g. pangeo-forge/staged-recipes#183 (comment).

We should add testing against a feedstock that uses a preprocessor function, to catch these issues early.

To do this, I suggest we create a branch on our test feedstock https://github.com/pforgetest/gpcp-from-gcs-feedstock that implements a basic (even no-op) preprocessor, and then parametrize test_bake with that branch here:

pangeo-forge-runner/tests/test_bake.py

Lines 63 to 65 in 36380ff

	"https://github.com/pforgetest/gpcp-from-gcs-feedstock.git",
	"--ref",
	"4f41e02512b2078c8bdb286368a1a9d878b5cec2",