Python: Add test showing taint for attr store

In `x.arg = TAINTED_STRING` there is a store step to the attribute `arg`
of `x`. In our taint modeling, we allow _any_ store step with the code
below. This means that we also say there is a taint-step directly from
`TAINTED_STRING` to `x` :|

```codeql
  // construction by literal
  // TODO: Not limiting the content argument here feels like a BIG hack, but we currently get nothing for free :|
  DataFlowPrivate::storeStep(nodeFrom, _, nodeTo)
```

python/ql/test/experimental/dataflow/tainttracking/defaultAdditionalTaintStep/test_attr.py

@@ -0,0 +1,43 @@
+# Add taintlib to PATH so it can be imported during runtime without any hassle
+import sys; import os; sys.path.append(os.path.dirname(os.path.dirname((__file__))))
+from taintlib import *
+
+# This has no runtime impact, but allows autocomplete to work
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from ..taintlib import *
+
+
+# Actual tests
+
+class Foo:
+    def __init__(self, arg):
+        self.arg = arg
+        self.other_arg = "other_arg"
+
+
+def test_tainted_attr():
+    # The following demonstrates how tainting an attribute affected the taintedness of
+    # the object.
+    #
+    # Previously we would (wrongly) treat the object as tainted if we noticed a write of
+    # a tainted value to any of its' attributes. This lead to FP, highlighted in
+    # https://github.com/github/codeql/issues/7786
+
+    f = Foo(TAINTED_STRING)
+    ensure_not_tainted(f) # $ SPURIOUS: tainted
+    ensure_tainted(f.arg) # $ tainted
+    ensure_not_tainted(f.other_arg)
+
+
+    x = Foo("x")
+    ensure_not_tainted(x, x.arg, x.other_arg)
+
+    x.arg = TAINTED_STRING
+    ensure_not_tainted(x) # $ SPURIOUS: tainted
+    ensure_tainted(x.arg) # $ tainted
+    ensure_not_tainted(f.other_arg)
+
+
+    b = Foo("bar")
+    ensure_not_tainted(b, b.arg, b.other_arg)