Merge #16589: build: Remove unused src/obj-test folder

fa4ee0d build: Remove unused src/obj-test folder (MarcoFalke)

Pull request description:

  I believe this is no longer used after we switched from a makefile to autotools

ACKs for top commit:
  emilengler:
    Concept ACK bitcoin/bitcoin@fa4ee0d

Tree-SHA512: cbedd3e65afc4de93b16500a0ff2e8778eb94e0224409d9a8d7cfc14cf22087471d260e63ba8d1322afdef5930d6c26fb64060e9e1b23876132cb9cdc35dce2c

Author: MarcoFalke