CLI: Add --seed=true to generate a new seed

[Krinkle]

• Change --seed to require a value.
  This fixes the conflict with test file arguments, raised in CLI --seed without value and files argument is counterintuitive #1691.
• Add --seed=true to generate a new seed.
  This makes it consistent with URL Params, where ?seed (which translated to seed=true) already did this. With this patch, we now also support pre-config qunit_config_seed=true and qunit_config_seed="true". As well as --seed=true on the CLI. And URL param ?seed=true (in case someone tries/expects that).

While at it, this also:

• Remove duplicate logic from CLI for generating new seed value in value of /src/core/config.js, by translating --seed to a preconfig.

Fixes #1691.