pr-auditor: support test plans in poorly formatted markdown

Created by: bobheadxi

Previously the regexp required the test plan anchor to be at the start of a line, which fails on PRs with poorly formatted markdown. This changes the regexp to allow the test plan anchor to show up anywhere

Fun example: https://github.com/sourcegraph/sec-pr-audit-trail/issues/170, the PR https://github.com/sourcegraph/deploy-sourcegraph-cloud/pull/16102 has a test plan but it starts after some leading whitespace

Test plan

unit test