Update README.developers.md : Added sanitizer suppression file info, and mention of valgrind.

[vaughnbetz]

Described valgrind and the sanitizer suppression files in the developer debugging aids documentation.

Related Issue

We didn't document these, so many people didn't know how to run them properly outside the CI system.

Checklist:

• [ x] My change requires a change to the documentation
• [ x] I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[vaughnbetz]

CI of the sanitized runs look OK in this run which is good.
@sdamghan : these lines at the end of the Odin II basic test look a bit scary. Is this a problem (looks like a regtest script had an error)?

2021-07-29T16:10:00.2861669Z ==========================================================================
2021-07-29T16:10:00.2863617Z Verilog-to-Routing Regression Testing
2021-07-29T16:10:00.2864932Z Traceback (most recent call last):
2021-07-29T16:10:00.2867099Z File "run_reg_test.py", line 342, in
2021-07-29T16:10:00.2867622Z ==========================================================================
2021-07-29T16:10:00.2868676Z vtr_command_main(sys.argv[1:])
2021-07-29T16:10:00.2869526Z File "run_reg_test.py", line 179, in vtr_command_main
2021-07-29T16:10:00.2870335Z vtr_task_list_files = collect_task_list(reg_test)
2021-07-29T16:10:00.2871168Z File "run_reg_test.py", line 300, in collect_task_list
2021-07-29T16:10:00.2872050Z raise IOError("Test does not exist: {}".format(reg_test))
2021-07-29T16:10:00.2872942Z OSError: Test does not exist: ../../../ODIN_II/regression_test/run001/vtr

The test concludes that it passed, but I'm wondering if those lines mean some tests didn't actually run.

[sdamghan]

@vaughnbetz that is actually because currently vtr_reg_multiclock does not exist in regression_tests. Once PR #1807 merged, the message will disappear.
Also with the my commit in PR #1807 the CI test won't pass similar cases.

[vaughnbetz]

Thanks @sdamghan