fix(agent): Ignore startup-errors in test mode

[srebhan]

Summary

When running Telegraf with --test startup errors of plugins are supposed to be logged and the plugin to be ignored. However, currently the plugin is added to the processing units despite reporting errors on startup. Therefore, the Gather() method of those erroneous plugins is called leading to problems due to partial startup (or no startup at all). As an example the following configuration

[[inputs.openstack]]
  username = "admin"
  password = "password"

will cause Telegraf to panic because parts of the internal plugin structure are not initialized but Gather() is called.

The PR ignores plugins reporting startup errors and makes Telegraf to behave as expected in testing mode.

Checklist

• No AI generated code was used in this PR

Related issues

[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 doesn't significantly change the Telegraf binary size (less than 1%)

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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