ts: basic documentation for utility scripts

Onnx conformance tests

* Add ONNX conformance tests

* dnn(test): add filters for ONNX conformance tests

* add filter lists for OCV backend

* address review comments

* move test_clip_inbounds to all_denylist

* address clip issue

* avoid empty lists
Co-authored-by: NAlexander Alekhin <alexander.a.alekhin@gmail.com>

If tests are run through GTest option `--gtest_also_run_disabled_tests`

- added functionality to collect memory usage of OpenCL sybsystem
- memory usage of fastMalloc() (disabled by default):
  * It is not accurate sometimes - external memory profiler is required.
- specify common `CV_TEST_TAG_` macros
- added applyTestTag() function
- write memory usage / enabled tags into Google Tests output file (.xml)

TS: fix Python v2/v3 compatibility (#12667)

* TS: fix Python2.7 compatibility

* TS: fix Python3 compatibility

* py3: use integer division '/' => '//' instead of cast

- moved to table title
- can be restored via '--show_units' option
- fix microseconds: mks -> us

Now it can determine, without looking at the file name, both the module
name and the configuration name (the latter with a little help from the
configuration file).

Also, removed the one from modules/python/src2/cv.py and cleared its
executable bit, since it's not a script.