Merge cascaded failure feature dependencies together.

[BillyONeal]

Before:

.\vcpkg.exe install vcpkg-depends-on-fail[x] --overlay-ports C:\Dev\vcpkg-tool\azure-pipelines\e2e-ports --keep-going --binarysource clear
[...]
error: building vcpkg-depends-on-fail:x64-windows failed with: CASCADED_DUE_TO_MISSING_DEPENDENCIES
  due to the following missing dependencies:
    vcpkg-fail-if-depended-upon[a]:x64-windows
    vcpkg-fail-if-depended-upon[core]:x64-windows
    vcpkg-fail-if-depended-upon[b]:x64-windows
    vcpkg-fail-if-depended-upon[core]:x64-windows

After:

.\vcpkg.exe install vcpkg-depends-on-fail[x] --overlay-ports C:\Dev\vcpkg-tool\azure-pipelines\e2e-ports --keep-going --binarysource clear
[...]
error: building vcpkg-depends-on-fail:x64-windows failed with: CASCADED_DUE_TO_MISSING_DEPENDENCIES
  due to the following missing dependencies:
    vcpkg-fail-if-depended-upon[a,b,core]:x64-windows

I discovered this upon seeing that there is a duplicate implementation here I'm removing:

vcpkg-tool/src/vcpkg/commands.test-features.cpp

Line 496 in d3138cb

std::string failed_dependencies;

[BillyONeal]

Note that this does not list is-a-default-feature even though we tried to install it here because that isn't actually a dependency of vcpkg-depends-on-fail