New user account for installing command line software
I can see two questions there.
- Is it best to use yet another account for command line software?
You don't say what command line software, but may well be quite pointless as the installation may place the software for use by all users. For example, the Homebrew package manager places software under /usr/local.
- Would deleting the 'test' account clean everything up?
Not unless the software was installed under /Users/test.
As the tool you want is installed via conda this makes some difference.
First conda itself sandboxes things to some extent (It alters the path for different environments) . Set up a new conda environment and then install Spleeter in that, unless you switch to that environment the tool will not be on your path and so can't be run (easily)
If you really want to be safe then you could set up a test user. All the things installed by conda will be in the test user's home directory. For testing and if you are worried this might be the best.
For things not installed via conda e.g. by a standard Unix ./configure;make install see the other answer but better use a package manager e.g. Macports, Homebrew etc. where someone else will have worked out how to install the tool.
For both cases if you are really worried then install macOS in a Virtual Machine and then install into that