[lldb] Improve assert message in TestCPPAccelerator

`log` is just some IO object that gets printed as `<_io.TextIOWrapper = filename`
but the intention here was to print the actual found log contents.

lldb/test/API/lang/cpp/accelerator-table/TestCPPAccelerator.py

@@ -23,11 +23,12 @@ class CPPAcceleratorTableTestCase(TestBase):
         # of it not being in the first file looked at.
         self.expect('frame variable inner_d')
 
-        log = open(logfile, 'r')
+        with open(logfile) as f:
+            log = f.readlines()
         n = 0
         for line in log:
             if re.findall(r'[abcdefg]\.o: FindByNameAndTag\(\)', line):
                 self.assertIn("d.o", line)
                 n += 1
 
-        self.assertEqual(n, 1, log)
+        self.assertEqual(n, 1, "".join(log))