JosmDecimalFormatSymbolsProviderTest fails on Ubuntu with Java 11 and 14

[Stereo]

​https://github.com/grischard/josm/runs/814121328?check_suite_focus=true reports an error in the Ubuntu 11 and 14 runs. Windows and macOS are unaffected. The problem doesn't happen on Java 8.

From Java 14 on ubuntu-latest with headless=true:

Tests run: 4, Failures: 3, Skipped: 0, Aborted: 0, Time elapsed: 8 sec(s)
Testcase: org.openstreetmap.josm.tools.JosmDecimalFormatSymbolsProviderTest
Test: testParseDouble took 588 milli sec(s)
Test: testGroupingSeparator took 581 milli sec(s) FAILED: de expected:<123[ ]456> but was:<123[.]456>
org.junit.ComparisonFailure: de expected:<123[ ]456> but was:<123[.]456>
	at org.openstreetmap.josm.tools.JosmDecimalFormatSymbolsProviderTest.checkGroupingSymbol(JosmDecimalFormatSymbolsProviderTest.java:42)
	at org.openstreetmap.josm.tools.JosmDecimalFormatSymbolsProviderTest.testGroupingSeparator(JosmDecimalFormatSymbolsProviderTest.java:36)
	at org.openstreetmap.josm.testutils.JOSMTestRules$TimeoutThread.run(JOSMTestRules.java:765)

[simon04]

Closed as duplicate of #19888.
Closing this older ticket one as duplicate since the other ticket has accumulated more details…