Add reference test case for --cover assume code.

This was provided as reference for an issue described in
#6057.

Author: NlightNFotis

regression/cbmc-cover/assume_assert1/assume_assert.c

@@ -0,0 +1,9 @@
+#include <assert.h>
+
+int main()
+{
+  int x;
+  __CPROVER_assume(x > 0);
+  __CPROVER_assume(x < 0);
+  assert(0 == 1);
+}

regression/cbmc-cover/assume_assert1/test.desc

@@ -0,0 +1,11 @@
+CORE
+assume_assert.c
+--cover assume
+^EXIT=0$
+^SIGNAL=0$
+^\[main.coverage.1\] file assume_assert.c line \d function main assert\(false\) before assume\(x > 0\): SATISFIED$
+^\[main.coverage.2\] file assume_assert.c line \d function main assert\(false\) after assume\(x > 0\): SATISFIED$
+^\[main.coverage.3\] file assume_assert.c line \d function main assert\(false\) before assume\(x < 0\): SATISFIED$
+^\[main.coverage.4\] file assume_assert.c line \d function main assert\(false\) after assume\(x < 0\): FAILED$
+--
+^warning: ignoring