[qa] Use single cache dir for chains

qa/pull-tester/rpc-tests.py

@@ -192,6 +192,7 @@ def runtests():
         coverage = RPCCoverage()
         print("Initializing coverage directory at %s\n" % coverage.dir)
     flags = ["--srcdir=%s/src" % BUILDDIR] + passon_args
+    flags.append("--cachedir=%s/qa/cache" % BUILDDIR)
     if coverage:
         flags.append(coverage.flag)