Enable TestConvenienceVariables on Linux

The test has passed in the last 300 runs for me, enabling to see what the buildbot says.

llvm-svn: 236498

lldb/test/functionalities/embedded_interpreter/TestConvenienceVariables.py

@@ -20,7 +20,6 @@ class ConvenienceVariablesCase(TestBase):
     @dwarf_test
     @skipIfFreeBSD # llvm.org/pr17228
     @skipIfRemote
-    @expectedFailureLinux("llvm.org/pr20276") # intermittent failure on Linux
     @expectedFailureWindows("llvm.org/pr22274: need a pexpect replacement for windows")
     def test_with_dwarf_and_run_commands(self):
         """Test convenience variables lldb.debugger, lldb.target, lldb.process, lldb.thread, and lldb.frame."""