test: improve BDB parser (handle internal/overflow pages, support all page sizes)

[theStack]

This PR adds missing features to our test framework's BDB parser with the goal of hopefully being able to read all legacy wallets that are created with current and past versions of Bitcoin Core. This could be useful both for making review of #26606 easier and to also possibly improve our functional tests for the wallet BDB-ro parser by additionally validating it with an alternative implementation. The second commits introduces a test that create a legacy wallet with huge label strings (in order to create overflow pages, i.e. pages needed for key/value data than is larger than the page size) and compares the dump outputs of wallet tool and the extended test framework BDB parser.
It can be exercised via $ ./test/functional/tool_wallet.py --legacy. BDB support has to be compiled in (obviously).

For some manual tests regarding different page sizes, the following patch can be used:

diff --git a/src/wallet/bdb.cpp b/src/wallet/bdb.cpp
index 38cca32f80..1bf39323d3 100644
--- a/src/wallet/bdb.cpp
+++ b/src/wallet/bdb.cpp
@@ -395,6 +395,7 @@ void BerkeleyDatabase::Open()
                             DB_BTREE,                                 // Database type
                             nFlags,                                   // Flags
                             0);
+            pdb_temp->set_pagesize(1<<9); /* valid BDB pagesizes are from 1<<9 (=512) to <<16 (=65536) */
 
             if (ret != 0) {
                 throw std::runtime_error(strprintf("BerkeleyDatabase: Error %d, can't open database %s", ret, strFile));

I verified that the newly introduced test passes with all valid page sizes between 512 and 65536.

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Type	Reviewers
Concept ACK	laanwj, BrandonOdiwuor

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[laanwj]

Code review ACK, from what i learned about how BDB databases work internally, implementation looks correct on first glance.

[theStack]

Rebased on master (necessary since #26606 was merged and also touched the same functional test file).

[BrandonOdiwuor]

Concept ACK

[rkrux]

Incomplete review and untested yet, left couple comments off the top of my head atm.
Will do another round thoroughly in the next few days.

[rkrux]

There's a separate effort from @maflcko regarding removing the usage of struct.pack from all places in test. Though here it's struct.unpack but I guess the reasoning for the removal of struct.pack is still applicable.
#29401

[maflcko]

There is a small benefit to struct.pack being less code when more than one item is packed or unpacked. Seems fine to use either in this case, up to the author. However, in most other cases where only a single item is (un)packed, the modern alternatives are less code, and clearer.

[rkrux]

WDYT about creating a constant tuple for these values? They seem pretty generic enough.