Python: Add copy of internal Python 3 tests

Again, mostly extractor tests, and a single library test.

python/ql/test/3/extractor-tests/fstrings3.8/FString.ql

@@ -0,0 +1,8 @@
+
+import python
+
+from Fstring str, int n, Expr e
+where e = str.getValue(n) and
+not exists(FormattedValue v | v.getFormatSpec() = str)
+select str.getLocation().getStartLine(), str.toString(), n, e.toString()
+