Add classpath option to float to string test and mark it as KNOWNBUG

The test previously passed by accident as it did not specify the models jar via
the classpath option of jbmc. Thus, the models were not loaded and the string
operations were nondet'ed, and verification failed as required by the test.

Author: danpoe

jbmc/regression/jbmc-strings/StringValueOf09/test.desc

@@ -1,6 +1,6 @@
-CORE
+KNOWNBUG
 StringValueOf09.class
---max-nondet-string-length 1000
+--max-nondet-string-length 1000 --cp `../../../../scripts/format_classpath.sh . ../../../lib/java-models-library/target/core-models.jar`
 ^EXIT=10$
 ^SIGNAL=0$
 ^\[.*assertion\.1\] .* line 7 .* FAILURE$