Prior to this commit, tool_bitcoin.py was failing:
```sh
$ ./bld-cmake/test/functional/tool_bitcoin.py --valgrind
TestFramework (ERROR): Unexpected exception
Traceback (most recent call last):
File "./test/functional/test_framework/test_framework.py", line 138, in main
self.setup()
~~~~~~~~~~^^
File "./test/functional/test_framework/test_framework.py", line 269, in setup
self.setup_network()
~~~~~~~~~~~~~~~~~~^^
File "./test/functional/tool_bitcoin.py", line 38, in setup_network
assert all(node.args[:len(node_argv)] == node_argv for node in self.nodes)
~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError
```
This commit fixes this issue by running `bitcoin` under valgrind. Also,
it comes with other improvements:
* Drop the outdated valgrind 3.14 requirement, because there is no
distro that ships a version that old anymore.
* Drop the VALGRIND_SUPPRESSIONS_FILE env var handling, because it was
presumably never used since it was introduced. Also, the use-case
seems limited.
Review note:
The set_cmd_args was ignoring the --valgrind test option.
In theory, this could be fixed by refactoring Binaries::node_argv() to
be used here. However, for now, just re-implement the node_argv logic in
set_cmd_args to prepend the valgrind cmd.
