lib: fix overflowing fromHexString tests and example (#433710)

lib/tests/misc.nix

@@ -386,8 +386,9 @@ runTests {
     expected = 255;
   };
 
-  testFromHexStringSecondExample = {
-    expr = fromHexString (builtins.hashString "sha256" "test");
+  # Highest supported integer value in Nix.
+  testFromHexStringMaximum = {
+    expr = fromHexString "7fffffffffffffff";
     expected = 9223372036854775807;
   };
 
@@ -396,6 +397,30 @@ 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 = [

lib/trivial.nix

@@ -1114,7 +1114,7 @@ in
     fromHexString "FF"
     => 255
 
-    fromHexString (builtins.hashString "sha256" "test")
+    fromHexString "0x7fffffffffffffff"
     => 9223372036854775807
     ```
   */