fab17e8272f5f70213f186809479ee7a75898b1d test: Add basic test for BIP34 (MarcoFalke)
Pull request description:
BIP34 was disabled for testing, which explains why it had no test.
Fix that by enabling it and adding a test.
Tree-SHA512: 9cb5702d474117ce6420226eb93ee09d6fb5fc856fabc8b67abe56a088cd727674e0e5462000e1afa83b911374036f90abdbdde56a8c236a75572ed47e10a00f
Backport of Core [[https://github.com/bitcoin/bitcoin/pull/15059 | PR15059]]