Split functional testings via github action matrix

This commit changes the workflow of the github actions.

We split the workflow into different parts:

1) build zfs modules for Ubuntu 20.04 and 22.04 (~25m)
2) 2x zloop test (~10m) + 2x sanity test (~25m)
3) functional testings in parts 1..5 (each ~1h)
   - these could be triggered, when sanity tests are ok
   - currently I just start them all in the same time
4) cleanup and create summary

When everything is fine, the full run with all testings
should be done in around 2 hours.

The codeql.yml and checkstyle.yml are not part in this circle.

The testings are also modified a bit:
- report info about CPU and checksum benchmarks
- reset the debugging logs for each test
  - when some error occurred, we call dmesg with -c to get
    only the log output for the last failed test
  - we empty also the dbgsys

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #14078

.github/workflows/README.md

@@ -0,0 +1,51 @@
+
+## The testings are done this way
+
+```mermaid
+flowchart TB
+subgraph CleanUp and Summary
+  Part1-20.04-->CleanUp+nice+Summary
+  Part2-20.04-->CleanUp+nice+Summary
+  PartN-20.04-->CleanUp+nice+Summary
+  Part1-22.04-->CleanUp+nice+Summary
+  Part2-22.04-->CleanUp+nice+Summary
+  PartN-22.04-->CleanUp+nice+Summary
+end
+
+subgraph Functional Testings
+  functional-testing-20.04-->Part1-20.04
+  functional-testing-20.04-->Part2-20.04
+  functional-testing-20.04-->PartN-20.04
+  functional-testing-22.04-->Part1-22.04
+  functional-testing-22.04-->Part2-22.04
+  functional-testing-22.04-->PartN-22.04
+end
+
+subgraph Sanity and zloop Testings
+  sanity-checks-20.04-->functional-testing-20.04
+  sanity-checks-22.04-->functional-testing-22.04
+  zloop-checks-20.04-->functional
+  zloop-checks-22.04-->functional
+end
+
+subgraph Code Checking + Building
+  codeql.yml
+  checkstyle.yml
+  Build-Ubuntu-20.04-->sanity-checks-20.04
+  Build-Ubuntu-22.04-->sanity-checks-22.04
+  Build-Ubuntu-20.04-->zloop-checks-20.04
+  Build-Ubuntu-22.04-->zloop-checks-22.04
+end
+```
+
+
+1) build zfs modules for Ubuntu 20.04 and 22.04 (~15m)
+2) 2x zloop test (~10m) + 2x sanity test (~25m)
+3) functional testings in parts 1..5 (each ~1h)
+4) cleanup and create summary
+   - content of summary depends on the results of the steps
+
+When everything runs fine, the full run should be done in
+about 2 hours.
+
+The codeql.yml and checkstyle.yml are not part in this circle.