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So I have debugged the first failure and it has to do with the sign extending of 'signed char' parameters. The first test case basically does this:
 _dll = CDLL(_ctypes_test.__file__)
 _dll.tf_b.restype = c_byte
 _dll.tf_b.argtypes = (c_byte,)
 _dll.tf_b(-126)
where 'tf_b' is defined in C code like:
 signed char tf_b(signed char c) { return c/3; }
Clang with -O3 generates code for 'tf_b' like:
0x0000000100000ed0 <tf_b+0>:	push %rbp
0x0000000100000ed1 <tf_b+1>:	mov %rsp,%rbp
0x0000000100000ed4 <tf_b+4>:	movslq %edi,%rax
0x0000000100000ed7 <tf_b+7>:	imul 0ドルx55555556,%rax,%rax
0x0000000100000ede <tf_b+14>:	mov %rax,%rcx
0x0000000100000ee1 <tf_b+17>:	shr 0ドルx3f,%rcx
0x0000000100000ee5 <tf_b+21>:	shr 0ドルx20,%rax
0x0000000100000ee9 <tf_b+25>:	add %ecx,%eax
0x0000000100000eeb <tf_b+27>:	movsbl %al,%eax
0x0000000100000eee <tf_b+30>:	pop %rbp
0x0000000100000eef <tf_b+31>:	retq 
See how 'movslq' is used to sign extend the first argument? Since the first argument is a 'signed char', that should be 'movsbq'. I am pretty sure this is a clang bug.
I am going to see if this is fixed in later versions of clang. I will also search the clang tracker to see if this has been reported.