[tests] add cover demo

[Clo91eaf]

#837

Add coverpoint support to the simplest mlir.hello test case as a cover demo.

• Added hello.json file at the same level as the source code to describe coverpoints.
• Modified default.nix build script to use jq to parse hello.json into URG coverage hierarchy format.

[sequencer]

Please add a documentation about how to add coverage points, tests.

[sequencer]

How do you align the RTL coverpoint(from LTL) to this name?

[Clo91eaf]

There are actually two rules here:

1. When Chisel lowers to sv, it will replace . in the label of LTL in the Chisel code with _.
2. VCS enables the assertions based on wildcard matching of the name in the full design:
   If a full hierarchical name is provided, such as A.C, vcs will determine the matching label. If only the last name is provided, such as C, vcs will still match the same label A.C. It is worth noting that if If there is another name, such as B.C, then vcs will match both A.C and B.C. And Some wildcard operation are also allowed, refer to the Coverage Technology Reference Manual of VCS
   So the 'vmv_v_i' here actually corresponds to the full hierachy name 'TestBench.dut.verification.vmv_v_i'