Add builtins._ doctest

Defining builtins._ should not break anything outside the doctest while also should be working as expected inside the doctest.

IPython/testing/plugin/test_ipdoctest.py

@@ -74,3 +74,19 b' def doctest_multiline3():'
     In [15]: h(0)
     Out[15]: -1
    """
+
+
+def doctest_builtin_underscore():
+    """Defining builtins._ should not break anything outside the doctest
+    while also should be working as expected inside the doctest.
+
+    In [1]: import builtins
+
+    In [2]: builtins._ = 42
+
+    In [3]: builtins._
+    Out[3]: 42
+
+    In [4]: _
+    Out[4]: 42
+    """