Remove a few usages of old version-spec approach from test-suite

Testsuite: Replace --version=x.y.z with package@x.y.z

Make 'dub remove' remove all packages by default in non-interactive mode ...

Given how often it is used in the test-suite, it should be self-obvious
that it will be what most users want.

testsuite: Document what interactive remove is doing and make it reproducible ...

It's a terrible solution, but it works for the time being.

Add dub fetch <package>:<version> shortcut

GitHub's API + UI don't return 0.9.20 anymore

Add support for DragonFlyBSD ...

- Switch from /bin/sh to /usr/bin/env bash
- Ignore temporary and test-build files (gitignore)
- Untrack changes to source/dub/version_.d
- Handling 'isDragonflybsd'
- Use 'tr -d' instead of 'tr --deleted' which works on Linux and BSD based platforms
- Fix FRONTEND = unbound variable (test/run-unittest.sh)

replace all usages of `exit` ...

- those skipped the ERR trap
- add die function to exit with line number and message

unify flags and error handling ...

- also add trap ERR handler to report exact error location

silence confusing test output ...

- failure is expected and should not appear in logs as it's easily
  confused to be the cause for a failed test

split remove into callback- and version-selection overload ...

- make remove w/ --version non-interactive

use 1 based index for selection ...

- format similar to bash's select

interactive package removal ...

- ask user which version of a package to remove