[infer] Fix empty `issues.exp.test` on Ant build test

Summary:
There was an issue on `make build_ant_test` that newly generated `issues.exp.test` is empty.  When
infer runs with `-- ant`, if there is already built objects in `ant_out`, infer analyzes nothing,
which result in the empty `issues.exp.test`.  For example,

```
~/infer/infer/tests/build_systems/ant$ make test
[11:26:49][36924] Testing ant integration...
[      2s][36924] SUCCESS Testing ant integration

~/infer/infer/tests/build_systems/ant$ touch `which infer`
```

The makefile tries to reanalyze ant targets because `infer` binary has been changed.  However, there
is nothing to build/analyze, since `ant_out` still exists as the same.

```
~/infer/infer/tests/build_systems/ant$ make test
[11:26:57][37078] Testing ant integration...
[      1s][37078] SUCCESS Testing ant integration
 diff --git a/Users/scho/infer/infer/tests/build_systems/ant/issues.exp b/Users/scho/infer/infer/tests/build_systems/ant/issues.exp.test
index 376146b5f..e69de29bb 100644
 --- a/Users/scho/infer/infer/tests/build_systems/ant/issues.exp
+++ b/Users/scho/infer/infer/tests/build_systems/ant/issues.exp.test
@@ -1 +0,0 @@
[-build_systems/ant/src/Hello.java, Hello.test():int, 2, NULL_DEREFERENCE, no_bucket, ERROR, [start of procedure test()]-]

Test output (build_systems/ant/issues.exp.test) differs from expected test output build_systems/ant/issues.exp
Run the following command to replace the expected test output with the new output:

  make -C build_systems/ant replace

make: *** [test] Error 1
```

To handle this issue, this cleans `ant_out` before running infer.

Reviewed By: jvillard

Differential Revision: D21950916

fbshipit-source-id: f57056f6e

infer/tests/build_systems/ant/Makefile

@@ -19,4 +19,4 @@ $(OBJECTS): $(SOURCES)
 
 infer-out/report.json: $(JAVA_DEPS) $(SOURCES)
 	$(QUIET)$(call silent_on_success,Testing ant integration,\
-	  $(INFER_BIN) --project-root $(TESTS_DIR) -- $(ANT))
+	  $(ANT) clean && $(INFER_BIN) --project-root $(TESTS_DIR) -- $(ANT))