Testsuite: Fix calls to die to always include line numbers ...

'die' actually taking two parameters seems to be a little-known fact.

unify flags and error handling ...

- also add trap ERR handler to report exact error location

use DC instead of COMPILER ...

- so it's clear that it can be dmd/ldc2/gdc

mktemp on OSX requires a template

Get rid of jq checks for now, as Travis doesn't seem to support it.

Simplify some things. Use jq to test some of the dub describe output.

Add --import-paths and --string-import-paths as options for 'dub describe' instead of a new command. Add some scripts for testing the output of each, and a smoke test for 'dub describe'.