add options for other valgrind tools in MTR

Using other valgrind tools in MTR is a little difficult as the MTR ties the tool to memcheck or calllgrind.

https://github.com/MariaDB/server/pull/194 provides a valgrind-tool option for easy selection.

MTR_TEST_NAME is also exported for recording results per test run (using a custom --valgrind-path= wrapper script).