[BUGFIX] Drop only bootstrap-opened output buffers

[sbuerk]

TYPO3 v15 removes the implicit output buffer that Bootstrap::init() opened as
its very first action. Two places in this package compensated for that buffer
with an unconditional ob_end_clean():

• Testbase::setUpBasicTypo3Bootstrap() — called from both branches of
  FunctionalTestCase::setUp(), i.e. once per test
• FunctionalTestCase::executeFrontendSubRequest() — the "somewhere an
  ob_start() is called in frontend" of that comment is the
  Bootstrap::init() call a few lines above

Without the bootstrap buffer these calls close phpunit's own output buffer
instead, and phpunit reports every functional test as risky with
Test code or tested code closed output buffers other than its own.

Both places now remember ob_get_level() before Bootstrap::init() and unwind
only the buffers opened above that level. Deleting the calls outright was not an
option: branch main supports typo3/cms-core: 14.*.*@dev || 15.*.*@dev, and
TYPO3 v14 Bootstrap::init() still opens the buffer — a plain removal would
flip the failure mode on v14 to …did not close its own output buffers. The
while loop additionally guards the frontend sub request against stray buffers
left open by extension code, which is what the old "work around that for now"
comment was really about.

Reproduction

TYPO3 Core main with https://review.typo3.org/c/Packages/TYPO3.CMS/+/94863
cherry-picked, before this change:

Build/Scripts/runTests.sh -s functional -d sqlite typo3/sysext/core/Tests/Functional/Database/
→ Tests: 376, Assertions: 1309, Skipped: 34, Risky: 376

Build/Scripts/runTests.sh -s functional -d sqlite typo3/sysext/frontend/Tests/Functional/Rendering/TypeRenderingTest.php
→ Tests: 9, Assertions: 9, Risky: 9

Core CI for that change fails in all functional jobs (sqlite 3/3, mariadb 6/6,
postgres 10/10) while unit, phpstan, cgl, lint, integration various,
documentation rendering and the playwright e2e jobs stay green.

Core test runs with this change

vendor/typo3/testing-framework replaced by this branch.

TYPO3 v15 — Core main @ 3c6390b797a + Gerrit 94863/1 cherry-picked,
PHP 8.5, sqlite:

runTests.sh -s functional -d sqlite \
  typo3/sysext/core/Tests/Functional/Database/ \
  typo3/sysext/frontend/Tests/Functional/Rendering/TypeRenderingTest.php \
  typo3/sysext/frontend/Tests/Functional/Rendering/UriPrefixRenderingTest.php
→ Tests: 389, Assertions: 1338, Skipped: 34 — SUCCESS (0 risky)

runTests.sh -s functional -d sqlite typo3/sysext/core/Tests/Functional/
→ Tests: 4224, Assertions: 56298, Skipped: 36 — SUCCESS (0 risky)

TYPO3 v14 — Core 14 @ dd5735757cb (bootstrap does open the buffer),
PHP 8.2, sqlite:

runTests.sh -s functional -d sqlite \
  typo3/sysext/core/Tests/Functional/Database/ \
  typo3/sysext/frontend/Tests/Functional/Rendering/TypeRenderingTest.php \
  typo3/sysext/frontend/Tests/Functional/Rendering/UriPrefixRenderingTest.php
→ Tests: 384, Assertions: 1317, Skipped: 34 — SUCCESS (0 risky)

Package quality gates

Build/Scripts/runTests.sh in this repo, PHP 8.2 (default):

Gate	Result
-s composerUpdate	SUCCESS
-s cgl -n	SUCCESS (0 of 64 files to fix)
-s lint	SUCCESS
-s phpstan	SUCCESS (no errors)
-s unit	SUCCESS (127 tests, 337 assertions)

Notes

No backport needed: branches 9/8/7 do not support TYPO3 v15, so their
bootstrap always opens the buffer and the current unconditional ob_end_clean()
stays correct there.

Resolves: #732
Related: https://forge.typo3.org/issues/110250
Related: https://review.typo3.org/c/Packages/TYPO3.CMS/+/94863
Releases: main