Fix JVM target in tests using boolean comparisons

These tests don't work on JVM target 1.6 after 6d664bcd10 because we're
generating `Boolean.compare` which is only available since 1.8. (It is
not a big deal because JVM target 1.6 is prohibited for users now.)

compiler/testData/codegen/box/operatorConventions/compareTo/boolean.kt

@@ -1,3 +1,5 @@
+// JVM_TARGET: 1.8
+
 fun checkLess(x: Boolean, y: Boolean) = when {
     x >= y    -> "Fail $x >= $y"
     !(x < y)  -> "Fail !($x < $y)"