Render locale and plugin-bundled text as DokuWiki syntax

Static "intro" text shipped with DokuWiki, templates and plugins —
inc/lang/*/*.txt, template locale files, each plugin's lang/<lc>/*.txt —
i

Render locale and plugin-bundled text as DokuWiki syntax

Static "intro" text shipped with DokuWiki, templates and plugins —
inc/lang/*/*.txt, template locale files, each plugin's lang/<lc>/*.txt —
is authored in DokuWiki syntax. It is a core/plugin asset, not user
content. When an admin configures the wiki for Markdown the DW link and
monospace modes are not loaded, so these files render as literal text:
[[wiki:syntax]] and ''Save'' pairs survive into the HTML.

Pin those entry points to the 'dw' flavour via the override added in the
previous commit:

- p_locale_xhtml() and tpl_locale_xhtml() pass 'dw'.
- PluginTrait::locale_xhtml() passes 'dw'.
- PluginTrait::render_text() / PluginInterface::render_text() gain a
$syntax parameter defaulting to 'dw'. The default is 'dw', not null,
because the method predates GFM and its callers pass DW-syntax strings;
plugins rendering user content opt back into the configured syntax with
render_text($text, 'xhtml', null).

Locale output is now deterministic across a syntax switch, so its caches
get over-invalidated but never under-invalidated — acceptable, as the
locale render path is rare and cheap.

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Move PSR compliant tests to the apropriate namespaces

Having "inc" in the namespace is awkward. Instead the test class
namespaces now correspond to their real class namespaces.

With further refacto

Move PSR compliant tests to the apropriate namespaces

Having "inc" in the namespace is awkward. Instead the test class
namespaces now correspond to their real class namespaces.

With further refactorings we should get rid of most of the stuff in
tests/inc

this is a continuation of #3812

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