Move regression tests of the C front-end to a suitable folder [blocks: #2310, #3627]

[tautschnig]

There is no bounded model checking expected or required by these tests, and also
they use GCC-specific built-ins. There is still work left to be done, so most
remain KNOWNBUG (but now the reason thereof is explained).

Note that the set of tests marked CORE actually failed on Windows, but no one noticed because of a bug in our buildspec. PR to address this to follow.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 827d536).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/95528351

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 25de48d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96030402