fix: move `g_txindex` initialization out of erroneous location and into constructor

Motivation

g_txindex should be initialized in TestChainSetup's constructor but when backporting bitcoin#19806 (dash#5236), portions of the constructor were split into TestChainSetup::mineBlocks(), g_txindex's init was left behind in the latter instead of the former.

This meant that every mineBlocks() call would re-create a TxIndex instance, which is not intended behaviour; and was recorded to cause heap-use-after-frees (comment, also the reason this PR was opened).

This PR aims to resolve that.

Additional Information

Crashes stemming from previous attempts (except for one attempt) were not reproducible with my regular local setup (depends built with Clang 16, Dash Core built with Clang 16, set of debug-oriented flags, unit tests run using ./src/test/test_dash).
- Attempting to rebuild Dash Core with GCC 9 was insufficient, required to rebuild depends with GCC 9 as well
- configure'd with CC=gcc CXX=g++ CPPFLAGS="-DDEBUG_LOCKORDER -DARENA_DEBUG" ./configure --prefix=$(pwd)/depends/x86_64-pc-linux-gnu --enable-zmq --enable-reduce-exports --enable-crash-hooks --enable-c++20 --disable-ccache
- Unit tests must be run with make check-recursive -j$(( $(nproc --all) - 2 ))
An index must be initialized after the chain is constructed, this seems to be corroborated by all other index usage (source, source, source, all three use Start() for their respective indexes after TestChain100Setup's constructor runs mineBlocks())
- Attempting to run Start() earlier (before the mineBlocks() call in the constructor) results in erratic behaviour
- This also explains why my attempt at moving it back to TestingSetup (a grandparent of TestChainSetup) failed
Interrupt() is supposed to be called before Stop() but this was erroneously removed in a commit that adopted IndexWaitSynced. This has since been resolved.
In line with other indexes, an sanity check has been added. Additionally, as TxIndex::Start() is more akin to CChainState::LoadGenesisBlock() than CChainState::CanFlushToDisk(), the assert has been downgraded to an exception.

Checklist:

[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas (note: N/A)
[x] I have added or updated relevant unit/integration/functional/e2e tests (note: N/A)
[x] I have made corresponding changes to the documentation (note: N/A)
[x] I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

dashpay / dash