[Locale] Fixed: StubLocale::setDefault() throws no exception when "en" is passed
merged branch bschussek/fix-locale-2.2 (PR #8846)

This PR was merged into the 2.2 branch.

Discussion
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[2.2][Locale] Fixed: StubLocale::setDefault() throws no exception when "en" is passed

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This is necessary to fix `IntlTestHelper::requireIntl()` when ICU is not installed and the Icu component 1.0.0 is used instead. In that case, `IntlTestHelper::requireIntl()` calls `\Locale::setDefault('en')` and fails. Consequently, the Form component test cases are partially red when running without ICU installed.

This PR fixes that.

Commits
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d0faf55 [Locale] Fixed: StubLocale::setDefault() throws no exception when "en" is passed