binman: Fix up removal of temporary directories

At present 'make check' leaves some temporary directories around. Part of
this is because we call tools.PrepareOutputDir() twice in some cases,
without calling tools.FinaliseOutputDir() in between.

Fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/elf_test.py b/tools/binman/elf_test.py
index c16f714..b68530c 100644
--- a/tools/binman/elf_test.py
+++ b/tools/binman/elf_test.py
@@ -10,6 +10,7 @@
 
 import elf
 import test_util
+import tools
 
 binman_dir = os.path.dirname(os.path.realpath(sys.argv[0]))
 
@@ -46,6 +47,10 @@
 
 
 class TestElf(unittest.TestCase):
+    @classmethod
+    def setUpClass(self):
+        tools.SetInputDirs(['.'])
+
     def testAllSymbols(self):
         """Test that we can obtain a symbol from the ELF file"""
         fname = os.path.join(binman_dir, 'test', 'u_boot_ucode_ptr')