Feature/adjust estimated multitest exec time based on testcase count

[zhenyu-ms]

Bug / Requirement Description

to get better estimation of multitest exec time for improved test turnover time (more parts if more testcases) or better testing resource utilization (fewer parts if fewer testcases)

Solution description

allow extra field, i.e. total testcase count of previous run called testcase_count in runtime data file; during multitest auto-partitioning, estimated overall exec time will be adjusted by curr testcase count over prev testcase count, adjusted exec time will be written back to testrunner.cfg.runtime_data

update some type hints as well

Checklist:

• Test
• Example (both test_plan.py and .rst)
• Documentation (API)
• News fragment present for release notes
• MS info leakage check
• For new driver: driver index page
• For new assertion: ui/pdf/std renderers, documentation
• For new cmdline arg: documentation