Create smoke,regress,nightly tests; Add nightly GitHub workflow

[dhower-qc]

Regression tests are taking too long. This PR splits testing into smoke, regress, and nightly.

• smoke is basic validation tests (no generation)
• regress is needed to merge a PR. It contains smoke + generates one of each type of output.
• nightly is regress + builds all outputs.

This also adds a nightly run on GitHub.

NOTE This changes some of the task names to get more uniformity. Notably do validate is now do test:smoke.

The full list of documented tasks is:

hu-dhower-lv:riscv-unified-db-3 (90) ./do --tasks
do clean                                # Clean up all generated files
do gen:adoc[config_name]                # Generate Asciidoc source for config into gen/CONFIG_NAME/adoc
do gen:arch                             # Generate a unified architecture file (configuration independent) / Generate architecture files from layouts
do gen:cert_model_pdf[cert_model_name]  # Generate certificate documentation for a specific version as a PDF
do gen:cert_profile_pdfs                # Generate all certificates and profile PDFs
do gen:cfg_arch[config_name]            # Generate the cfg-specific architecture files for config_name
do gen:cfg_ext_html[extension,cfg]      # Generate HTML documentation for :extension that is defined or overlayed in :cfg
do gen:cfg_ext_pdf[extension,cfg]       # Generate PDF documentation for :extension that is defined or overlayed in :cfg
do gen:ext_pdf[extension]               # Generate PDF documentation for :extension
do gen:html[config_name]                # Generate HTML documentation for config(s)
do gen:html_manual                      # Generate an HTML site for one or more versions of the manual (./do --desc for options)
do gen:index                            # Generate index of the database
do gen:profile[profile_release]         # Create a specification PDF for +profile_release+
do gen:profile_html[profile_release]    # Create a specification HTML for +profile_release+
do gen:tool_doc                         # Generate documentation for the ruby tooling
do serve:html[config_name,port]         # Start an HTML server to view the generated HTML documentation for config_name
do serve:tool_doc[port]                 # Start an HTML server to view the generated HTML documentation for the tool
do test:nightly                         # Run the nightly regression tests
do test:regress                         # Run the regression tests
do test:smoke                           # Run smoke tests

[AFOliveira]

I think this is great, it definitely improves workflow!

[kbroch-rivosinc]

I agree the existing regress action was taking nearly 1/2 hr.