trapme: If using a TC custom target, ensure the CUSTOM_TARGET field is valid

From July TCK retrospective issue, there have been issues with people manually making a mess of the `CUSTOM_TARGET` field.
It would be useful for the harness to check for two specific situations which have been hit by multiple team members (at least myself and @steelhead31):
- If the CUSTOM_TARGET field has a prefix of `CUSTOM_TARGET=` which it should have
- If the CUSTOM_TARGET field is missing the `testSuite=` at the end of the field.

Start by locating trapme's handling of the CUSTOM_TARGET field and trace how its value is parsed. Add checks for the CUSTOM_TARGET= prefix and the trailing testSuite= field, then verify that both malformed cases are reported or rejected by the harness.

Written by the indexing model from the issue text.