lib: add fromHexString tests for distressing behaviour

This was cherry‐picked from
<https://github.com/NixOS/nixpkgs/pull/266705> and merged as part of
<https://github.com/NixOS/nixpkgs/pull/318712>, despite there being
a blocking review on the former pointing out these kinds of issues.

This documents some of the dodgy behaviour. It also can’t handle
negative literals. It might be worth considering deprecating and
dropping this, by inlining it into `lib.network.ipv6.fromString`,
its only in‐tree user.

lib/tests/misc.nix

@@ -390,6 +390,30 @@ runTests {
     expected = 15;
   };
 
+  # FIXME: This might be bad and should potentially be deprecated.
+  testFromHexStringQuestionableMixedCase = {
+    expr = fromHexString "eEeEe";
+    expected = 978670;
+  };
+
+  # FIXME: This is probably bad and should potentially be deprecated.
+  testFromHexStringQuestionableUnderscore = {
+    expr = fromHexString "F_f";
+    expected = 255;
+  };
+
+  # FIXME: This is definitely bad and should be deprecated.
+  testFromHexStringBadComment = {
+    expr = fromHexString "0 # oops";
+    expected = 0;
+  };
+
+  # FIXME: Oh my god.
+  testFromHexStringAwfulInjection = {
+    expr = fromHexString "1\nwhoops = {}";
+    expected = 1;
+  };
+
   testToBaseDigits = {
     expr = toBaseDigits 2 6;
     expected = [