Add initial implementation of skipping verilator build

[Kuree]

• Add a skip verilator option so that TBG can reuse existing binary. In TBG it will be used together with skip_compile=True to skip the magma circuit, verilog, and verilator compilation.
• Add a test to confirm the functionality.

[coveralls]

Pull Request Test Coverage Report for Build 827

• 4 of 5 (80.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 77.203%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
fault/verilator_target.py	4	5	80.0%
<!--	Total:	4	5	80.0%	-->

Totals
Change from base Build 819:	0.01%
Covered Lines:	1253
Relevant Lines:	1623

---

💛 - Coveralls