tests/import/builtin_ext.py: Add test for built-in module override.

This verifies the behavior:
 - Exact matches of built-ins bypass filesystem.
 - u-prefix modules can be overridden from the filesystem.
 - Builtin import can be forced using either u-prefix or sys.path=[].

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/import/builtin_ext.py

@@ -0,0 +1,36 @@
+import sys
+
+print(sys, hasattr(sys, "__file__"))
+
+sys.path.clear()
+sys.path.append("ext")
+
+# All three should only get builtins, despite sys.py, usys.py, and
+# micropython.py being in the path.
+import micropython
+
+print(micropython, hasattr(micropython, "__file__"))
+import sys
+
+print(sys, hasattr(sys, "__file__"))
+import usys
+
+print(usys, hasattr(usys, "__file__"))
+
+# This should get os.py, which uses uos to get the builtin.
+import os
+
+print(os, hasattr(os, "__file__"), os.sep, os.extra)
+
+# This should get time.py, which uses empty sys.path to get the builtin.
+import time
+
+print(time, hasattr(time, "__file__"), time.sleep, time.extra)
+
+# These should get the builtins.
+import uos
+
+print(uos, hasattr(uos, "__file__"), hasattr(uos, "extra"))
+import utime
+
+print(utime, hasattr(utime, "__file__"), hasattr(utime, "extra"))

tests/import/builtin_ext.py.exp

@@ -0,0 +1,10 @@
+<module 'sys'> False
+<module 'micropython'> False
+<module 'sys'> False
+<module 'sys'> False
+os from filesystem
+<module 'os' from 'ext/os.py'> True / 1
+time from filesystem
+<module 'time' from 'ext/time.py'> True <function> 1
+<module 'uos'> False False
+<module 'utime'> False False

tests/import/ext/micropython.py

@@ -0,0 +1,2 @@
+# micropython is always builtin and cannot be overriden by the filesystem.
+print("ERROR: micropython from filesystem")

tests/import/ext/os.py

@@ -0,0 +1,5 @@
+print("os from filesystem")
+
+from uos import *
+
+extra = 1

tests/import/ext/sys.py

@@ -0,0 +1,2 @@
+# sys is always builtin and cannot be overriden by the filesystem.
+print("ERROR: sys from filesystem")

tests/import/ext/time.py

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+print("time from filesystem")
+
+# Tests the CPython-compatible / non-u-prefix way of forcing a builtin
+# import.
+import sys
+
+_path = sys.path[:]
+sys.path.clear()
+from time import *
+
+sys.path.extend(_path)
+del _path
+
+extra = 1

tests/import/ext/usys.py

@@ -0,0 +1,3 @@
+# usys (and any u-prefix) is always builtin and cannot be overriden by the
+# filesystem.
+print("ERROR: usys from filesystem")