binman: Allow preserving test directories

Sometimes when debugging tests it is useful to keep the input and output
directories so they can be examined later. Add an option for this and
update the binman tests to support it. This affects both the test class
and the tearDown() function called after each test.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README b/tools/binman/README
index 7eda244..2f4c7fe 100644
--- a/tools/binman/README
+++ b/tools/binman/README
@@ -731,6 +731,14 @@
 being used (-T) since they are incompatible.
 
 
+Debugging tests
+---------------
+
+Sometimes when debugging tests it is useful to keep the input and output
+directories so they can be examined later. Use -X or --test-preserve-dirs for
+this.
+
+
 Advanced Features / Technical docs
 ----------------------------------