[qa] Use single cache dir for chains

.gitignore

@@ -103,10 +103,9 @@ linux-build
 win32-build
 qa/pull-tester/run-bitcoind-for-test.sh
 qa/pull-tester/tests_config.py
-qa/pull-tester/cache/*
 qa/pull-tester/test.*/*
 qa/tmp
-cache/
+qa/cache/*
 share/BitcoindComparisonTool.jar
 
 !src/leveldb*/Makefile