ffi/posix: add basic tests to sanity check cdefs

Ensure intermediate generated `ffi-cdecl/posix_h_XXX.lua` files can be loaded.

Author: benoit-pierre

spec/unit/posix_h_spec.lua

@@ -0,0 +1,23 @@
+local lfs = require "lfs"
+
+describe("generated", function()
+    local ffi_cdecl_dir = lfs.symlinkattributes("ffi").target .. "-cdecl"
+
+    for variant in string.gmatch([[
+        android_arm
+        android_arm64
+        android_x64
+        android_x86
+        linux_arm
+        linux_arm64
+        linux_x64
+        macos
+        ]], "[^%s]+") do
+        local file = string.format("%s/posix_h_%s.lua", ffi_cdecl_dir, variant)
+
+        it(file.." should load", function()
+            local ret = os.execute("./luajit "..file)
+            assert.are.equal(0, ret)
+        end)
+    end
+end)