tools: make convert-group ignore converted test files

Teach the tool that rewrites tests files to use _begin_fstest group
tagging to ignore tests that have already been treated.  This will make
it easier for people to rebase their dev branches.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tools/convert-group

@@ -16,6 +16,8 @@ obliterate_group_file() {
 		elif [ ! -e "$test" ]; then
 			echo "Ignoring unknown test file \"$test\"."
 			continue
+		elif grep -q '^_begin_fstest' "$test"; then
+			continue
 		fi
 
 		# Replace all the open-coded test preparation code with a