Improve specfun but still some failures 5 files

[ckormanyos]

The purpose of the PR is to further improve the output of the specfuntests.

We still do, however, have 5 files failing specfun.

Failing functions include:

• test_ibeta_inv_1.cpp (unkonwn, check exponentiation).
• test_ibeta_3.cpp ( maybe, ... also exponential function for large/small arguments).
• test_erf.cpp (most probably exponential function in extreme case).
• test_bessel_k.cpp (one test case out of the range of the backend type).
• test_carlson_2.cpp (many unknown errors, reason completely unknown, but text outputs now more meaningful).

Fahad (@sinandredemption) I'll try to isolate example(s) of each kind of test failure for detailed analysis soon.

Let's see if this thing passes the other parts of CI.

[ckormanyos]

The result is not worse and likely slightly better. Will merge.

Cc: @sinandredemption