Mark performance-varying test as THOROUGH

This test used to be THOROUGH but then seemed to be quick. Changes to
the sequence of equations, however, may adversely affect the runtime
with MiniSat.

Author: tautschnig

jbmc/regression/jbmc-strings/StringIndexOf/test_thorough.desc

@@ -1,7 +1,13 @@
-CORE
+THOROUGH
 Test
---function Test.check --unwind 10 --no-unwinding-assertions --max-nondet-string-length 10 --java-assume-inputs-non-null
+--function Test.check --unwind 10 --no-unwinding-assertions --max-nondet-string-length 10 --java-assume-inputs-non-null --sat-solver cadical
 ^EXIT=0$
 ^SIGNAL=0$
 ^VERIFICATION SUCCESSFUL$
 --
+--
+This test completes in less than 3 seconds with CaDiCaL
+--
+This test completes in less than 3 seconds with CaDiCaL, but may flip between
+seconds and several minutes with MiniSat even just changing the order of
+equations.