* CI: Intel icc/icpc via oneAPI

Add testing for Intel icc/icpc via the oneAPI images.
Intel oneAPI is in a late beta stage, currently shipping
oneAPI beta09 with ICC 20.2.

* CI: Skip Interpreter Tests for Intel

Cannot find how to add this, neiter the package `libc6-dev` nor
`intel-oneapi-mkl-devel` help when installed to solve this:
```
-- Looking for C++ include pthread.h
-- Looking for C++ include pthread.h - not found
CMake Error at /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find Threads (missing: Threads_FOUND)
Call Stack (most recent call first):
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindThreads.cmake:234 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  tests/test_embed/CMakeLists.txt:17 (find_package)
```

* CI: libc6-dev from GCC for ICC

* CI: Run bare metal for oneAPI

* CI: Ubuntu 18.04 for oneAPI

* CI: Intel +Catch -Eigen

* CI: CMake from Apt (ICC tests)

* CI: Replace Intel Py with GCC Py

* CI: Intel w/o GCC's Eigen

* CI: ICC with verbose make

* [Debug] Find core dump

* tests: use arg{} instead of arg() for Intel

* tests: adding a few more missing {}

* fix: sync with @tobiasleibner's branch

* fix: try ubuntu 20-04

* fix: drop exit 1

* style: clang tidy fix

* style: fix missing NOLINT

* ICC: Update Compiler Name

Changed upstream with the last oneAPI release.

* ICC CI: Downgrade pytest

pytest 6 does not capture the `discard_as_unraisable` stderr and
just writes a warning with its content instead.

* Use new test pinning requirements.txt

* tests: add notes about intel, cleanup

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>