[multiple] clang -Wstrict-prototypes for C2x

adjustments to compile warnings-free with recent clang changes that
warn about potential behavior change for non-prototypes, including
generic function p

[multiple] clang -Wstrict-prototypes for C2x

adjustments to compile warnings-free with recent clang changes that
warn about potential behavior change for non-prototypes, including
generic function pointers e.g. int(*)()

x-ref:
https://discourse.llvm.org/t/rfc-enabling-wstrict-prototypes-by-default-in-c/60521
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2432.pdf
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2841.htm

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[tests] test stubs for http_header.c and http_kv.c

[tests] combine tests into test_common.c

combine common components tests into test_common.c

combine into a single executable to reduce compilation/link redundancy