tests/extmod/vfs_posix.py: Fix test on Android.

This commit makes a slight change to the vfs_posix test suite to let it
pass on Android.

On Android, non-root processes can perform most filesystem operations
only on a restricted set of directories.  The vfs_posix test suite
attempted to enumerate the filesystem root directory, and said directory
happens to be restricted for non-root processes.  This would raise
an EACCES OSError and terminate the test with a unexpected failure.

To fix this, rather than enumerating the filesystem root directory the
enumeration target is the internal shared storage area root - which
doesn't have enumeration restrictions for non-root processes.  The path
is hardcoded because it is guaranteed to be there on pretty much any
recent-ish device for now (it stayed the same for more than a decade for
compatibility reasons).  The proper way would be to query the storage
subsystem via a JNI round-trip call, but this introduces too much
complexity for something that is unlikely to break going forward.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

tests/extmod/vfs_posix.py

@@ -29,7 +29,21 @@ print(os.getcwd() == curdir)
 print(type(os.stat("/")))
 
 # listdir and ilistdir
-print(type(os.listdir("/")))
+target = "/"
+try:
+    import platform
+
+    # On Android non-root users are permitted full filesystem access only to
+    # selected directories.  To let this test pass on bionic, the internal
+    # user-accessible storage area root is enumerated instead of the
+    # filesystem root.  "/storage/emulated/0" should be there on pretty much
+    # any recent-ish device; querying the proper location requires a JNI
+    # round-trip, not really worth it.
+    if platform.platform().startswith("Android-"):
+        target = "/storage/emulated/0"
+except ImportError:
+    pass
+print(type(os.listdir(target)))
 
 # mkdir
 os.mkdir(temp_dir)