Improve unittest build system (#13554)

Improve unittest build system (#13554)

* unittest - fix duplicate symbols in unittests

Some symbols were declared again
With clang -fcommon, this resulted in allocatin in common segment and
prevented error.
(tentative definitions in C standard).
-fno-common (now default in clang https://reviews.llvm.org/D75056)
causes compilation errors.

Declarations are now marked extern.

* unittest - fix scheduler array size for unittest

Unittest needs extra space for canary

* unittest - fix missing include (needed for clang-16)

* unittest - remove unused varibles

-Werror in clang 15+

* unittest - increase max supported version to clang-16

* unittest - conditionaly disable useless output in unittests

* unittest - C++11 version of STATIC_ASSERT

* unittest - fix initializers for g++

- Change order of initializers to match order in struct
- make valueTable initializion consistent (necessary for C++)
- adapt controlRateConfig

* unittest - adapt scheduler_unitest for g++

scheduler_stubs.c is necessary to initialize task_attributes

* unittest - fix ledstrip unittest

only part of config was zeroed

* unittest - fix g++ warnings

- memcpy when length is known and \0 is not copied
- isError is local stub, no extern
- serialReadStub - don't memcpy into object, use initializer

* cli - cleanup cliGetSettingIndex

- compare only passed bytes (old version may read data after
name)
- input string is const

* unittest - fix ld warning from PG sections

move pg data sections after .rodata. Sections were marked as writable
due to relocation (!?). That marked .text output section
(containing .pg_data) as writable too and linker correctly complained
that executable section is writable.

* unittest - cleanup

* unittest - adapt after code cleanup, add gcc

- remove clang flags that are not necessary now (tested on clang-11
and clang-16)
- add support for gcc ( make test CC=gcc CXX=g++ )
- add suport for different optimization level (detects some code
problems) : make test OPTIMIZE=-O2
- fallback to clang on Linux too

* fixup! unittest - conditionaly disable useless output in unittests