subsys/testsuite: rearrange for standard use of extern "C"

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

Backport: c0d33975
See: https://en.cppreference.com/w/cpp/language/language_linkage


Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>