do not explicitly set "en_US" locale in maven surefire tests

[tofi86]

From #916 (comment):

Currently the maven-surefire-plugin explicitly sets the locale in pom.xml:

<argLine>-Xmx1g -Duser.language=en -Duser.country=US</argLine>

However, now that we can define the locale with EPUBCheck 4.1 we should remove this, make it platform specific and set the locale explicitly in the tests where needed (Don't forget to reset to the system default after the test!)

Removing the -Duser.language=en -Duser.country=US results in the following errors on a german system:

Results :

Failed tests:
  OCFCheckerTest.testInvalidLoremBasic30:346 null
  OCFCheckerTest.testInvalidLoremBindings30:365 null
  OCFCheckerTest.testInvalidLoremForeign30:386 null
  OCFCheckerTest.testInvalidLoremRNG30:423 null
  UtilMessage_LocalizationTest.ensureDefault:23 Default constructor should use the (host) default locale. expected:<en> but was:<en_US>
  UtilMessage_LocalizationTest.ensureNullIsDefault:32 Null locale should use (host) default locale. expected:<en> but was:<en_US>
  api_Test.EpubCheck_InputStream_Test:76 Expected: ERROR: EPUB/lorem.opf(13,63): Spine item with non-standard media-type 'font/opentype' has no fallback. Actual: ERROR: EPUB/lorem.opf(13,63): Ein 'spine'-Eintrag mit MimeType 'font/opentype' ist nicht erlaubt und enthält auch kein Fallback. expected:<...B/lorem.opf(13,63): [Spine item with non-standard media-type 'font/opentype' has no f]allback.> but was:<...B/lorem.opf(13,63): [Ein 'spine'-Eintrag mit MimeType 'font/opentype' ist nicht erlaubt und enthält auch kein F]allback.>
  api_Test.EpubCheck_PrintWriter_Test:48 Expected: ERROR: EPUB/lorem.opf(13,63): Spine item with non-standard media-type 'font/opentype' has no fallback. Actual: ERROR: EPUB/lorem.opf(13,63): Ein 'spine'-Eintrag mit MimeType 'font/opentype' ist nicht erlaubt und enthält auch kein Fallback. expected:<...B/lorem.opf(13,63): [Spine item with non-standard media-type 'font/opentype' has no f]allback.> but was:<...B/lorem.opf(13,63): [Ein 'spine'-Eintrag mit MimeType 'font/opentype' ist nicht erlaubt und enthält auch kein F]allback.>

Tests run: 1027, Failures: 8, Errors: 0, Skipped: 1

So these are the tests which need to be fixed.

[rdeltour]

However, now that we can define the locale with EPUBCheck 4.1 we should remove this, make it platform specific and set the locale explicitly in the tests where needed

I would rather use Locale#getDefault() (to get the previous locale to reset in a @before method) and Locale.#setDefault(Locale) (to set the locale used in the test in an @after method), rather than the new EPUBCheck API…