This PR aims to add the groundwork to test the precision of libc complex
functions against MPC. I took `cargf` as a test to verify that the infra
works fine.
I have commented out the test for `neg_zero`(creal) because :
1. real(neg_zero + 0.0i) equals zero.
2. real(neg_zero - 0.0i) equals neg_zero.
I am not sure if this is the intended behaviour.
[EDIT]
I have updated tests for `neg_zero` (creal) to be :
```
EXPECT_FP_EQ(func(CFPT(neg_zero - zero * 1.0i)), neg_zero);
EXPECT_FP_EQ(func(CFPT(neg_zero + zero * 1.0i)), zero);
```
because all three [gcc, clang and GNU MPC] also give the same result.
https://godbolt.org/z/hxhcn6aof
and it seems that it is indeed the correct behaviour since Imaginary
types are not supported yet, refer #113671
