Move regression tests of the C front-end to a suitable folder

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).

Author: tautschnig

regression/cbmc/builtin_is/test.desc

@@ -0,0 +1,10 @@
+KNOWNBUG
+main.c
+
+^EXIT=1$
+^SIGNAL=0$
+expected to have two float/double arguments
+--
+--
+The C front-end should catch the problem during type checking, and not leave it
+to goto-program conversion to detect the problem.

regression/cbmc/builtin_va_copy_lvalue/test.desc

@@ -0,0 +1,10 @@
+KNOWNBUG
+main.c
+
+^EXIT=1$
+^SIGNAL=0$
+argument expected to be lvalue
+--
+--
+The C front-end should catch the problem during type checking, and not leave it
+to goto-program conversion to detect the problem.