Expand tests to reproduce #7655

The original `builtins.getContext` test from
1d757292d0 would have caught this. The
problem is that b30be6b450 adding
`builtins.appendContext` modified that test to make it test too much at
once, rather than adding a separate test.

We now have isolated tests for both functions, and also a property test
showing everything put together (in the form of an eta rule for strings
with context). This is better coverage and properly reproduces the bug.

tests/lang/eval-okay-context-introspection.exp

@@ -1 +1 @@
-true
+[ true true true true true true ]

tests/lang/eval-okay-context-introspection.nix

@@ -18,7 +18,24 @@ let
     };
   };
 
-  legit-context = builtins.getContext "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  combo-path = "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  legit-context = builtins.getContext combo-path;
 
-  constructed-context = builtins.getContext (builtins.appendContext "" desired-context);
-in legit-context == constructed-context
+  reconstructed-path = builtins.appendContext
+    (builtins.unsafeDiscardStringContext combo-path)
+    desired-context;
+
+  # Eta rule for strings with context.
+  etaRule = str:
+    str == builtins.appendContext
+      (builtins.unsafeDiscardStringContext str)
+      (builtins.getContext str);
+
+in [
+  (legit-context == desired-context)
+  (reconstructed-path == combo-path)
+  (etaRule "foo")
+  (etaRule drv.drvPath)
+  (etaRule drv.foo.outPath)
+  (etaRule (builtins.unsafeDiscardOutputDependency drv.drvPath))
+]