test: fs: Add fat12 to supported fs of some tests

The tests fs_ext, fs_mkdir and fs_unlink support fat12 without
modifications.
The fs_basic test uses a partition that is too large for fat12, so it
is omitted here.

Signed-off-by: Christian Taedcke <christian.taedcke@weidmueller.com>
diff --git a/test/py/tests/test_fs/conftest.py b/test/py/tests/test_fs/conftest.py
index 0d87d18..fa637a2 100644
--- a/test/py/tests/test_fs/conftest.py
+++ b/test/py/tests/test_fs/conftest.py
@@ -12,9 +12,9 @@
 from tests import fs_helper
 
 supported_fs_basic = ['fat16', 'fat32', 'ext4']
-supported_fs_ext = ['fat16', 'fat32']
-supported_fs_mkdir = ['fat16', 'fat32']
-supported_fs_unlink = ['fat16', 'fat32']
+supported_fs_ext = ['fat12', 'fat16', 'fat32']
+supported_fs_mkdir = ['fat12', 'fat16', 'fat32']
+supported_fs_unlink = ['fat12', 'fat16', 'fat32']
 supported_fs_symlink = ['ext4']
 
 #