tests/extmod: Add a test for core VFS functionality, sans any filesystem.

tests/extmod/vfs_basic.py

+# test VFS functionality without any particular filesystem type
+
+try:
+    try:
+        import uos_vfs as uos
+        open = uos.vfs_open
+    except ImportError:
+        import uos
+    uos.mount
+except (ImportError, AttributeError):
+    print("SKIP")
+    import sys
+    sys.exit()
+
+
+class Filesystem:
+    def __init__(self, id):
+        self.id = id
+    def mount(self, readonly, mkfs):
+        print(self.id, 'mount', readonly, mkfs)
+    def umount(self):
+        print(self.id, 'umount')
+    def listdir(self, dir):
+        print(self.id, 'listdir', dir)
+        return ['a%d' % self.id]
+    def chdir(self, dir):
+        print(self.id, 'chdir', dir)
+    def open(self, file, mode):
+        print(self.id, 'open', file, mode)
+
+
+# basic mounting and listdir
+uos.mount(Filesystem(1), '/test_mnt')
+print(uos.listdir())
+
+# referencing the mount point in different ways
+print(uos.listdir('test_mnt'))
+print(uos.listdir('/test_mnt'))
+
+# mounting another filesystem
+uos.mount(Filesystem(2), '/test_mnt2', readonly=True)
+print(uos.listdir())
+print(uos.listdir('/test_mnt2'))
+
+# chdir
+uos.chdir('test_mnt')
+print(uos.listdir())
+
+# open
+open('test_file')
+open('test_file', 'wb')
+
+# umount
+uos.umount('/test_mnt')
+uos.umount('/test_mnt2')

tests/extmod/vfs_basic.py.exp

+1 mount False False
+['test_mnt']
+1 listdir /
+['a1']
+1 listdir /
+['a1']
+2 mount True False
+['test_mnt', 'test_mnt2']
+2 listdir /
+['a2']
+1 chdir /
+1 listdir 
+['a1']
+1 open test_file r
+1 open test_file wb
+1 umount
+2 umount