Ensure mjsunit tests use dashes not underscores in flags directives.

test/mjsunit/regress/regress-2593.js

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-// Flags: --expose_gc
+// Flags: --expose-gc
 
 p1 =  { };
 p2 =  { };
 p3 =  { x : 1 };
 p2.__proto__ = p3
 p1.__proto__ = p2
 
 // Normalize p1.
 p1.z = 1
 delete p1.z
@@ skipping 13 matching lines @@
 for (var i = 0; i < 10; i++) f2();
 
 // Create load stub in p2 for p1.
 for (var i = 0; i < 10; i++) f1();
 
 assertEquals(1, f1());
 
 p2.x = 2;
 
 assertEquals(2, f1());