Zsh by default will error if a glob matches nothing (a smart default).
We need it to not error, and even better, to not use the Bash default of
using the pattern. Set nullglob to expand it to nothing.

Don't set the Bash version of nullglob in liquidprompt, since Bash
doesn't do local options well, and not at all until version 4.4. It is
safe to expand to a single string with the "*" in it, since the first
check we do is for the file existing, which it will not.

Add the sysfs power supply interface to the external-tool-tester.sh.

Some shells do not support the $'...' syntax, as it is still not a POSIX
standard. Dash is one such shell, which isn't a problem for
Liquidprompt, but is for this tester that I meant to write for any POSIX
shell.

But printf can handle these characters fine, even the \001, so replace
with that. I even tested it on Dash this time.

Add a check for the output being a terminal, and print help text if so.
Add a reminder at the end of the tests to upload the file.
Add more OS info detection.

This will collect example outputs of externals tools that Liquidprompt
uses, so that we can build integration tests for these tools.

This does not include VCS providers, since without a repository, most of
the commands will simply error out. Deeper integration tests will need
to be written for VCS providers. And while most of these tools here are
packaged with an OS and therefore are OS version dependent, VCS
providers are often updated to the latest version regardless of OS.