Remove uneeded dependency on tests.ipkg

Currently testing by doing:

git clean -fdx and rm -rf ~/.idris2 and a complete checkout on this branch with:

idris 1.3.2:git built on https://github.com/idris-lang/Idris-dev/pull/4846 with GMP and the following libraries on scope (the ones that are currently in idris-dev master):

idris --libdir                Sat 25 Apr 2020 03:38:27 PM CDT
/home/fabian/Development/Open/idris-dev/.stack-work/install/x86_64-linux-nix/1aa74e0bb7fad616b425a5617eb2fe78dc6bedb12b47f6c17c6bda32c3ae9d12/8.8.3/share/x86_64-linux-ghc-8.8.3/idris-1.3.2/libs/

idris-dev/.stack-work/install/x86_64-linux-nix/1aa74e0bb7fad616b425a5617eb2fe78dc6bedb12b47f6c17c6bda32c3ae9d12/8.8.3/share/x86_64-linux-ghc-8.8.3/idris-1.3.2/libs/
├── base
├── contrib
├── effects
├── prelude
└── pruviloj

nix-shell -p gmp.dev (to put gmp development libraries in scope for idris`
make install

EDIT: And it passes

edwinb / Idris2-boot

Remove uneeded dependency on tests.ipkg #323