Merge #12076: qa: Use node.datadir instead of tmpdir in test framework

c8330d4 qa: Use node.datadir instead of tmpdir in test framework (MarcoFalke)

Pull request description:

  Commit c53c9831ee introduced the utility function `get_datadir_path`, however not all places in the code use this util function. Using the util function everywhere makes it easier to review pull requests related to the datadir.

  This commit replaces datadir path creation with the `datadir` member of `TestNode`, which itself uses `get_datadir_path`.

Tree-SHA512: c75707ab7149d732a6d56152a5813138a33459d3d07577b60b89f2a207c83b7663fac5f203593677c9892d1c23a5eba4bd45c5c4ababf040d720b437240fcddf

test/functional/mempool_persist.py

@@ -93,8 +93,8 @@ class MempoolPersistTest(BitcoinTestFramework):
         self.start_node(0)
         wait_until(lambda: len(self.nodes[0].getrawmempool()) == 5)
 
-        mempooldat0 = os.path.join(self.options.tmpdir, 'node0', 'regtest', 'mempool.dat')
-        mempooldat1 = os.path.join(self.options.tmpdir, 'node1', 'regtest', 'mempool.dat')
+        mempooldat0 = os.path.join(self.nodes[0].datadir, 'regtest', 'mempool.dat')
+        mempooldat1 = os.path.join(self.nodes[1].datadir, 'regtest', 'mempool.dat')
         self.log.debug("Remove the mempool.dat file. Verify that savemempool to disk via RPC re-creates it")
         os.remove(mempooldat0)
         self.nodes[0].savemempool()