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().

Store last search direction when writing ShaDa files

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

Contains most of the logic