Make TestVSCode_step pass reliably

[labath]

Summary

The test was failing occasionally (1% of runs or so), because of
unpredictable timings between the two threads spawned by the test. If
the second thread hit the breakpoint right as we were stepping out of
the function on the first thread, we would still be stuck at the inner
frame when the process stopped.

This would cause errors like:

  File "/home/worker/lldb-x86_64-debian/lldb-x86_64-debian/llvm/tools/lldb/packages/Python/lldbsuite/test/tools/lldb-vscode/step/TestVSCode_step.py", line 67, in test_step
    self.assertEqual(x1, x3, 'verify step out variable')
AssertionError: 2 != 1 : verify step out variable

AFAICT, lldb-vscode is doing the right thing here, and the problem is
that the test is not taking this sequence of events into account. Since
the test is about testing stepping, it does not seem necessary to have
threads in the inferior at all, so I just rewrite the test to execute
the code we're supposed to step through directly on the main thread.

[jgorbe]

Thanks for the fix!

lldb/trunk/packages/Python/lldbsuite/test/tools/lldb-vscode/step/main.cpp

-#include <thread>
-
 int function(int x) {
   if ((x % 2) == 0)
     return function(x-1) + x; // breakpoint 1
   else
     return x;
 }
 
 int main(int argc, char const *argv[]) {
-  std::thread thread1(function, 2);
-  std::thread thread2(function, 4);
-  thread1.join();
-  thread2.join();
-  return 0;
+  return function(2);
 }