sysutils/node_exporter: Add tests, docs and examples

• Define LICENSE_FILE.
• Sort variables.
• Add a do-test target and prepare a testing environment. One test is skipped at the moment due to a problem with detecting procfs(5) on FreeBSD.
• Install documentation and examples.
• Change pkg-descr to better match upstream's README.

PR: 234791
Reviewed by: mat
Approved by: mat (mentor), maintainer timeout
Sponsored by: BALLY WULFF Games & Entertainment GmbH
Differential Revision: https://reviews.freebsd.org/D18792