WIP: Switch appveyor build from Release to Debug #20392

As per brief discussion. Visual Studio 2019 and Appveyor images have recently been updated. While it's only a minor update that doesn't break ABI compatibility it does seem to have broken compatibility for Release builds. This apparently occurs if compiler optimisations are turned on.

This PR switches the Appveyor CI job from a Release build to a Debug build in order to fix the CI failures.

Subsequent to this PR I will commence a new build of the Qt static libraries with the latest Visual Studio 2019 toolset so that release build can work again.

@hebasto fyi.

https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/36314660#L506:

C:\projects\bitcoin\src\test\net_tests.cpp(146): Entering test case "caddrdb_read_corrupted"
unknown location(0): fatal error: in "net_tests/caddrdb_read_corrupted": stack overflow

From past experience the msvc compiler can be quite aggressive regarding access to uninitialised/unallocated memory on debug builds. The unit test failure on caddrdb_read_corrupted is likely due to a debug-only assert.

The choices that occur to me are:

• Continue building as a Release build and update the pre-compiled Qt libraries for VS2019 v16.8.1.

  • Pros:
    • Unit tests will pass
  • Cons:
    • The same ABI incompatibility is likely to occur again for a future VS minor release. In the time Bitcoin Core has been using Appveyor (approx. 2 years I think) this is the first case.
    • New Qt 5.9.8 build required.

• Switch to a Debug build.

  • Pros;
    • ABI compatibility for VS 2019 minor releases should be preserved since debug builds do not use compiler optimisation flags which are only guaranteed to be compatible for libraries built with identical Visual Studio versions.
  • Cons:
    • Need to fix or disable unit tests causing debug asserts.

But why build failed?

C:\projects\bitcoin\src\test\net_tests.cpp(86): Leaving test suite "net_tests"; testing time: 140513us
Test is aborted
Leaving test module "Bitcoin Core Test Suite"; testing time: 1089996937us
*** 1 failure is detected in the test module "Bitcoin Core Test Suite"
Detected memory leaks!

Closing this PR as I've managed to get the msvc release build working again. Will open a new PR for that.