Make doh_unit_test and doh_ffi_test can be run on the R platfrom

To make the tests run in R platform, we need to have them statically link
libcrypto_static and libssl
1. Move doh_ffi_test into tests/.
2. doh_unit_test links libquiche_static instead of libquiche.
3. Minor cleanup, remove the useless soong var.

Bug: 155855709
Test: TH
Original-Change: https://android-review.googlesource.com/1711569
Merged-In: I158691c7e62696fe3613ad5841cab01b45354b6c
Change-Id: I158691c7e62696fe3613ad5841cab01b45354b6c