fix(tests): remove the __extension__ keyword in filter_complex_blocks (#32483)

Problem: This keyword is used by GCC and Clang to prevent -Wpedantic
(and other options) from emitting warnings for many GNU C extensions.
This is used heavily in Alpine Linux through musl libc and
foritfy-headers. Without filtering the __extension__ keyword some type
definitions are duplicated. For example, timeval is defined once as

  struct timeval { time_t tv_sec; suseconds_t tv_usec; };

and once as:

  __extension__ struct timeval { time_t tv_sec; suseconds_t tv_usec; };

Without this patch, the LuaJIT C parser doesn't recognize that these
definitions are equivalent, causing unit test to fail on Alpine Linux.

Solution: Filter out the keyword in filter_complex_blocks.

test/unit/testutil.lua

@@ -151,6 +151,9 @@ local function filter_complex_blocks(body)
         or string.find(line, 'mach_vm_range_recipe')
       )
     then
+      -- Remove GCC's extension keyword which is just used to disable warnings.
+      line = string.gsub(line, '__extension__', '')
+
       result[#result + 1] = line
     end
   end