test(inputs.libvirt): Merge mocking into test file

[srebhan]

Summary

This PR moves the libvirt mocking into the test file to prevent testing code to leak into the released binary.

Checklist

• No AI generated code was used in this PR
• AI generated code used in this PR follows the InfluxData Policy on AI-Generated Code Contributions

Related issues

resolves #19374

[skartikey]

@srebhan make check-deps is failing lint-linux: now that nothing outside the tests pulls testify in, the github.com/stretchr/objx and github.com/stretchr/testify entries in docs/LICENSE_OF_DEPENDENCIES.md need to go.

[skartikey]

@srebhan perfect!

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

⚠️ This pull request increases the Telegraf binary size by 6.53 % for linux amd64 (new size: 328.4 MB, nightly size 308.2 MB)

📦 Click here to get additional PR build artifacts

Artifact URLs

. DEB	. RPM	. TAR . GZ	. ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz