LibC: Allow multiple includes of <assert.h>

ISO C requires in section 7.2:
The assert macro is redefined according to the current state of NDEBUG
each time that <assert.h> is included.

Also add tests for `assert` multiple inclusion accordingly.

Meta/check-style.py

@@ -31,6 +31,7 @@ LICENSE_HEADER_CHECK_EXCLUDES = {
 # We check that "#pragma once" is present
 PRAGMA_ONCE_STRING = '#pragma once'
 PRAGMA_ONCE_CHECK_EXCLUDES = {
+    'Userland/Libraries/LibC/assert.h',
 }
 
 # We make sure that there's a blank line before and after pragma once