test: fix unit tests caused by error output being routed to stderr instead of stdout

[ykim-akamai]

📝 Description

Unit tests have been failing due to error output being routed to stderr instead of stdout introduced by this PR

The reason why it wasn't detected in the workflow in the first place is because our makefile returned status of export command instead of actual exit code of pytest command.
E.g.

.PHONY: testunit
testunit:
	@mkdir -p /tmp/linode/.config
	@orig_xdg_config_home=$${XDG_CONFIG_HOME:-}; \
	export LINODE_CLI_TEST_MODE=1 XDG_CONFIG_HOME=/tmp/linode/.config; \
	pytest -v tests/unit; \
	export XDG_CONFIG_HOME=$$orig_xdg_config_home;

This fix is also in this PR

✔️ How to Test

make testunit

📷 Preview

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[lgarber-akamai]

Tests are passing on my end, thank you for fixing this!

[ezilber-akamai]

Tests passing locally for me as well!