tests: add libfuzzer test target

Disabled by default, enabble with `meson -Dbuild_libfuzzer=true -Db_lundef=false ...`.
Fuzz target improved by the paralell work by Thierry Foucu in !138.
7 jobs for master in 3 minutes and 39 seconds
latest
Status Job ID Name Coverage
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passed #2371
debian amd64
build-debian

00:00:44

passed #2375
debian aarch64
build-debian-aarch64

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debian aarch64 clang5
build-debian-aarch64-clang-5

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debian amd64
build-debian-static

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macos
build-macos

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passed #2373
win32
build-win32

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win64
build-win64

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