[gravitypdf-bot]

Adds the one test #847 showed gravitypdf was missing: text after a Plane 2 character is still substituted when nothing of backupSIPFont is in the font cache yet (#828). No code changes, so no changelog entry.

gravitypdf-v6 covers this in SubstitutionFontCacheTest, added by #830. Here, the new test uses no font packages, so the fonts have no ranges files and each is read from the font itself:

• Text: Next 𠀀 then ⇒ ā in Garuda, from an empty font cache.
• Expected fonts: Garuda, Sun-ExtB, Garuda, the core Symbol font, DejaVu Sans Condensed.

Left out

Two other v6 test cases aren't ported, because gravitypdf already covers them:

• A font with a sip-ext font keeps the Plane 2 text it has: SubstitutionRunSplitTest::testPlaneTwoTextAFontWithASipExtFontHasStaysInIt.
• Core font text from an empty font cache: CoreFontBackupSubstitutionTest.

Testing

• The test catches the bug: with a temporary line in the substitution scan that gives up when the SIP font isn't loaded, as SubstituteCharsMB() stops substituting at a CJK Extension B character the first time its fallback font is used #828 did, only Garuda is drawn and the test fails.
• PHP 8.5: full suite passes, 13,488 tests with 31 skipped.
• composer cs: clean.
• PHP 5.6: the new file lints clean; the suite on 5.6 is left to CI.

/simplify wasn't run, as this is one short test that reuses the TempFontDirectory trait.