Use expr pass for positive soundness tests

The soundness check operates at the expr pass level. Positive tests
don't need LLVM codegen to verify the check doesn't reject valid code.
Stop at "expr" instead of "ir" to avoid hitting a pre-existing LLVM 21
codegen crash in genprim_string_serialise that only manifests in CI
debug builds.

Also reverts the Foo workaround — with expr pass, String iso tests
work correctly.

Author: SeanTAllen

test/libponyc/badpony.cc

@@ -1472,19 +1472,16 @@ TEST_F(BadPonyTest, MatchIsoCaptureWithConsume)
   // From issue #3596
   // Consuming the match expression makes the discriminee ephemeral,
   // so iso captures are sound.
-  // Uses a custom class instead of String to avoid hitting an unrelated
-  // LLVM 21 codegen issue in genprim_string_serialise.
   const char* src =
-    "class Foo\n"
-
     "actor Main\n"
     "  new create(env: Env) =>\n"
-    "    let x: Foo iso = recover iso Foo end\n"
+    "    let x: String iso = recover iso String end\n"
     "    match consume x\n"
-    "    | let y: Foo iso => None\n"
+    "    | let y: String iso => None\n"
     "    end";
 
-  TEST_COMPILE(src);
+  // Soundness check is in the expr pass; stop before codegen.
+  DO(test_compile(src, "expr"));
 }
 
 TEST_F(BadPonyTest, MatchNonIsoCaptureFromUnion)
@@ -1500,7 +1497,8 @@ TEST_F(BadPonyTest, MatchNonIsoCaptureFromUnion)
     "    | let s: String => None\n"
     "    end";
 
-  TEST_COMPILE(src);
+  // Soundness check is in the expr pass; stop before codegen.
+  DO(test_compile(src, "expr"));
 }
 
 TEST_F(BadPonyTest, MatchViewpointRefCaptureFromField)
@@ -1522,7 +1520,8 @@ TEST_F(BadPonyTest, MatchViewpointRefCaptureFromField)
     "actor Main\n"
     "  new create(env: Env) => None";
 
-  TEST_COMPILE(src);
+  // Soundness check is in the expr pass; stop before codegen.
+  DO(test_compile(src, "expr"));
 }
 
 TEST_F(BadPonyTest, MatchValConstraintCapture)
@@ -1544,7 +1543,8 @@ TEST_F(BadPonyTest, MatchValConstraintCapture)
     "actor Main\n"
     "  new create(env: Env) => None";
 
-  TEST_COMPILE(src);
+  // Soundness check is in the expr pass; stop before codegen.
+  DO(test_compile(src, "expr"));
 }
 
 TEST_F(BadPonyTest, MatchAliasedViewpointCapture)
@@ -1568,5 +1568,6 @@ TEST_F(BadPonyTest, MatchAliasedViewpointCapture)
     "actor Main\n"
     "  new create(env: Env) => None";
 
-  TEST_COMPILE(src);
+  // Soundness check is in the expr pass; stop before codegen.
+  DO(test_compile(src, "expr"));
 }