tests: Add test for print_exception() function.

tests/misc/print_exception.py

+import io
+import sys
+if hasattr(sys, 'print_exception'):
+    print_exception = sys.print_exception
+else:
+    import traceback
+    print_exception = lambda e, f: traceback.print_exception(None, e, None, file=f)
+
+try:
+    1/0
+except Exception as e:
+    print('caught')
+    buf = io.StringIO()
+    print_exception(e, buf)
+    s = buf.getvalue()
+    for l in s.split("\n"):
+        # uPy and CPy tracebacks differ in that CPy prints a source line for
+        # each traceback entry. In this case, we know that offending line
+        # has 4-space indent, so filter it out.
+        if not l.startswith("    "):
+            print(l)