tests/extmod: Make some vfs tests fully unmount FSs before running.

Otherwise the existing FSs can interfere with the tests, and in some
cases the tests can write to the real FS on the device.

tests/extmod/vfs_basic.py

@@ -47,6 +47,10 @@ class Filesystem:
 
 
 # first we umount any existing mount points the target may have
+try:
+    uos.umount('/')
+except OSError:
+    pass
 for path in uos.listdir('/'):
     uos.umount('/' + path)
 

tests/extmod/vfs_fat_more.py

@@ -44,6 +44,14 @@ except MemoryError:
     print("SKIP")
     sys.exit()
 
+# first we umount any existing mount points the target may have
+try:
+    uos.umount('/')
+except OSError:
+    pass
+for path in uos.listdir('/'):
+    uos.umount('/' + path)
+
 uos.VfsFat.mkfs(bdev)
 uos.mount(bdev, '/')