Activate tests for StringBuffer concat in loops

We activate these tests which check that refinement is not needed when
strings are not tested. They should all run in a reasonnable time now,
the most difficult takes about 2 seconds.

regression/jbmc-strings/StringConcat/test_buffer_nondet_loop.desc

@@ -1,4 +1,4 @@
-FUTURE
+CORE
 Test.class
 --refine-strings --string-max-length 1000 --string-max-input-length 100 --function Test.bufferNonDetLoop --depth 10000 --unwind 5  --verbosity 10  --property "java::Test.bufferNonDetLoop:(ILjava/lang/String;)Ljava/lang/String;.assertion.1"
 ^EXIT=10$

regression/jbmc-strings/StringConcat/test_buffer_nondet_loop2.desc

@@ -1,4 +1,4 @@
-FUTURE
+CORE
 Test.class
 --refine-strings --string-max-length 1000 --string-max-input-length 100 --function Test.bufferNonDetLoop2 --depth 10000 --unwind 5  --verbosity 10
 ^EXIT=10$

regression/jbmc-strings/StringConcat/test_buffer_nondet_loop4.desc

@@ -1,4 +1,4 @@
-FUTURE
+CORE
 Test.class
 --refine-strings --string-max-length 1000 --string-max-input-length 100 --function Test.bufferNonDetLoop4 --depth 10000 --unwind 5  --verbosity 10 --property "java::Test.bufferNonDetLoop4:(IILjava/lang/String;)Ljava/lang/String;.assertion.1"
 ^EXIT=10$

regression/jbmc-strings/StringConcat/test_buffer_nondet_loop5.desc

@@ -1,6 +1,6 @@
-FUTURE
+CORE
 Test.class
---refine-strings --string-max-length 1000 --string-max-input-length 100 --function Test.bufferNonDetLoop5 --depth 10000 --unwind 5  --verbosity 10
+--refine-strings --string-max-length 1000 --string-max-input-length 100 --function Test.bufferNonDetLoop5 --depth 10000 --unwind 3 --verbosity 10
 ^EXIT=10$
 ^SIGNAL=0$
 ^VERIFICATION FAILED$

regression/jbmc-strings/StringConcat/test_string_nondet.desc

@@ -1,4 +1,4 @@
-FUTURE
+CORE
 Test.class
 --refine-strings --string-max-length 1000 --string-max-input-length 100 --function Test.stringNonDet --depth 10000 --unwind 5 --verbosity 10
 ^EXIT=10$