Increase timeout of incremental loop unwinding tests

8 seconds seemed arbitrary, and we have now seen a failure on one
platform, which might be just down to jitter. Increase the timeout to 15
seconds, which should be plenty.

Author: tautschnig

regression/cbmc-incr-oneloop/Makefile

@@ -2,10 +2,10 @@ default: tests.log
 
 # Note the `perl -e` serves the purpose of providing timeout
 test:
-	@../test.pl -c "perl -e 'alarm shift @ARGV; exec @ARGV' 8 ../../../src/cbmc/cbmc --slice-formula"
+	@../test.pl -c "perl -e 'alarm shift @ARGV; exec @ARGV' 15 ../../../src/cbmc/cbmc --slice-formula"
 
 tests.log: ../test.pl
-	@../test.pl -c "perl -e 'alarm shift @ARGV; exec @ARGV' 8 ../../../src/cbmc/cbmc --slice-formula"
+	@../test.pl -c "perl -e 'alarm shift @ARGV; exec @ARGV' 15 ../../../src/cbmc/cbmc --slice-formula"
 
 clean:
 	@$(RM) *.log