fix stack overflow when building debug code

[dsyme]

Fixes #1410, a problem building the debug version of this repo with F# 6.

1. It turns out Debug build of FSharp.Data.Tests was busted for a while with F# 5 because tailcalls weren't being taken during JSON parsing.
2. When turning on explicit tailcalls for debug code, the problem went away with F# 5 but remains for F# 6 because of F# 6 takes fewer tailcalls in debug code when explicit /tailcalls+ is specified dotnet/fsharp#12413.
3. The code changes workaround this

Also adds a debug build to the PRs