Replace test_data/unit fixtures with txt-verified E2E tests

Constants and StaticFlag move to test_data with mains and expected
output generated by a real JVM. ConstantsReader is compiled against a
non-final Constants so javac emits getstatic instead of inlining,
keeping ConstantValue preparation observable from bytecode. The
Rust-side putstatic typed-read assertion is dropped with test_data/unit;
the narrowing fix itself is still exercised by StaticFlag's clinit.

Author: dlunch

test_data/Constants.class

@@ -1,4 +1,4 @@
-class Constants {
+public class Constants {
     static final boolean FLAG = true;
     static final byte B = 3;
     static final char C = 'a';
@@ -8,4 +8,14 @@ class Constants {
     static final float F = 1.5f;
     static final double D = 2.5;
     static final String STR = "hello";
+
+    public static void main(String[] args) {
+        System.out.println(FLAG);
+        System.out.println(B);
+        System.out.println(C);
+        System.out.println(S);
+        System.out.println(I);
+        System.out.println(L);
+        System.out.println(STR);
+    }
 }

test_data/unit/Constants.java test_data/Constants.javatest_data/unit/Constants.java renamed to test_data/Constants.java

@@ -0,0 +1,7 @@
+true
+3
+a
+7
+42
+1234567890123
+hello

test_data/Constants.txt

@@ -0,0 +1,13 @@
+// compiled against a non-final Constants so javac emits getstatic instead of
+// inlining the constants; run against the final Constants.class with ConstantValue attributes
+public class ConstantsReader {
+    public static void main(String[] args) {
+        System.out.println(Constants.FLAG);
+        System.out.println(Constants.B);
+        System.out.println(Constants.C);
+        System.out.println(Constants.S);
+        System.out.println(Constants.I);
+        System.out.println(Constants.L);
+        System.out.println(Constants.STR);
+    }
+}

test_data/ConstantsReader.class

@@ -0,0 +1,7 @@
+true
+3
+a
+7
+42
+1234567890123
+hello

test_data/ConstantsReader.java

@@ -0,0 +1,13 @@
+public class StaticFlag {
+    static boolean FLAG = true;
+    static byte SMALL = 3;
+    static char LETTER = 'a';
+    static short COUNT = 7;
+
+    public static void main(String[] args) {
+        System.out.println(FLAG);
+        System.out.println(SMALL);
+        System.out.println(LETTER);
+        System.out.println(COUNT);
+    }
+}

test_data/ConstantsReader.txt

@@ -0,0 +1,4 @@
+true
+3
+a
+7