Use unique filenames to avoid test conflicts.
Use read_file() instead of io.popen(), to ensures the file is closed.
Use helpers.rmdir(), it is far more robust than lfs.
closes#7911
It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.
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
