Tests fail without warning when Verilator not installed

**Type of issue**: feature request

**Impact**: no functional change

**Development Phase**: request

**What is the current behavior?**
When you run sbt test without having Verilator installed (for either Chisel or Firrtl), you get a large number of failures. It isn't clear why these failures are occurring (Verilator isn't installed) without looking through the logs.
**What is the expected behavior?**
It should either complain and exit early, or tell you at the end that errors may be because Verilator isn't installed (maybe even not run tests that depend on it).
**Please tell us about your environment:**
Forgot to install Verilator, macOS High Sierra.
**What is the use case for changing the behavior?**
Easier realising that you forgot to install Verilator.

Start by running `sbt test` without Verilator installed and inspect the resulting Chisel and Firrtl failures and logs. Trace where the Verilator-dependent tests are invoked; done means the missing dependency produces an early, clear message or an end-of-run explanation instead of unexplained failures.

Written by the indexing model from the issue text.