build_image: Factor out CheckHeadroom() and add tests.

The test is mostly trivial, but it ensures the result parsing from
actual calls to mke2fs.

Test: python -m unittest test_build_image
Test: `m dist` on aosp_marlin-userdebug (w/ and w/o defining
      PRODUCT_SYSTEM_HEADROOM respectively).
Change-Id: I8b9964213950e76f6d7d5518414a1bab888b4706