-NaN doesn't exist in the IEEE 754 spec, it is a hardware-specific detail
abstracted away by luajit(and not by lua or nvim), so there's no need to test
it. Normalize all tests that involve -nan so the suite will be compatible with
both Lua and Luajit.
The tests would leave the following test files in the root directory:
Xtest-functional-plugin-shada.shada
Xtest-functional-plugin-shada.shada.tmp.f
Clean them up in teardown().
Note: it looks like viminfo files do not store search direction intentionally.
After reading viminfo file search direction was considered to be “forward”.
Note 2: all files created on earlier Neovim version will automatically receive
“forward” direction.
Fixes#3580
