test: Fix broken --valgrind handling after bitcoin wrapper

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* Drop the outdated valgrind 3.14 requirement, because there is no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.

test/functional/tool_bitcoin.py

@@ -39,7 +39,9 @@ class ToolBitcoinTest(BitcoinTestFramework):
 
     def set_cmd_args(self, node, args):
         """Set up node so it will be started through bitcoin wrapper command with specified arguments."""
-        node.args = [self.binary_paths.bitcoin_bin] + args + ["node"] + self.node_options[node.index]
+        # Manually construct the `bitcoin node` command, similar to Binaries::node_argv()
+        bitcoin_cmd = node.binaries.valgrind_cmd + [node.binaries.paths.bitcoin_bin]
+        node.args = bitcoin_cmd + args + ["node"] + self.node_options[node.index]
 
     def test_args(self, cmd_args, node_args, expect_exe=None, expect_error=None):
         node = self.nodes[0]