repo: Add OSTREE_REPO_TEST_ERROR=invalid-cache env var

Add an invalid-cache test error flag to ensure that the code that checks
for and recovers from a corrupted summary cache is hit. This helps make
sure that the recovery path is actually used without resorting to
G_MESSAGES_DEBUG.

Closes: #1698
Approved by: cgwalters

src/libostree/ostree-repo-private.h

@@ -61,7 +61,8 @@ G_BEGIN_DECLS
 #define OSTREE_COMMIT_TIMESTAMP "ostree.commit.timestamp"
 
 typedef enum {
-  OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0)
+  OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0),
+  OSTREE_REPO_TEST_ERROR_INVALID_CACHE = (1 << 1),
 } OstreeRepoTestErrorFlags;
 
 struct OstreeRepoCommitModifier {