Extend TestDoProcessTx() in #105 for txs with multiple parents and children.

[xhliu]

#105

Similar to TestProcessBlock(), e.g.,
tx6 <- tx7 <- tx8 <- tx9
tx6 is not in main pool, so tx7/8/9 are orphaned. Add tx6 in main pool and utxo set, tx7/8/9 are also added.

Remove tx6 and ensure tx7/8/9 are orphaned again.

Different from blocks, a tx can have multiple children spending its multiple outputs, also multiple parents since it has multiple inputs spending from different txs, forming a Direct Acyclic Graph (DAG). E.g., tx has parents tx1 & tx2, children tx3 & tx4
tx1 tx2
\ /
tx
/ \
tx3 tx4
Do the aforementioned same test for txs with multiple parents and children.