Tests assume that error messages are always in English

[ZZZank]

Tests assumed that error messages are always in English, so when running tests in a system where system default language is not English, evaluations might fail:

org.junit.ComparisonFailure: `var a = 3` [MESSAGE] expected:<[missing ; after statement]> but was:<[在语句后面缺少 “;”]>
	at org.junit.Assert.assertEquals(Assert.java:117)
	at org.mozilla.javascript.tests.Bug789277Test.assertMessageEquals(Bug789277Test.java:453)
	at org.mozilla.javascript.tests.Bug789277Test.parserWithIdeErrorReporter_Reader_IdeMode(Bug789277Test.java:447)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)

Adding an option that forces using en_us lang in message formatting should be enough for fixing this i guess