buildman: Add a test for Boards.scan_defconfigs()

Add a test for this code. It requires some defconfig files and a test
Kconfig to work with, so copy these into the temporary directory at the
start.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/test/configs/board0_defconfig b/tools/buildman/test/configs/board0_defconfig
new file mode 100644
index 0000000..50e562e
--- /dev/null
+++ b/tools/buildman/test/configs/board0_defconfig
@@ -0,0 +1 @@
+CONFIG_TARGET_BOARD0=y