fix(installer-test): port InstallerTest::setUp's chdir(__DIR__)

PHP's InstallerTest::setUp() does chdir(__DIR__) before every test so
relative "type": "path" repository URLs in fixtures resolve against
composer/tests/Composer/Test; tearDown() restores the previous cwd.
This was never ported, only the env-var clears were, so any fixture
declaring a relative path repo failed at PathRepository::initialize()
with "the `url` supplied for the path (...) repository does not
exist" — not a missing PathRepository feature, just an unset cwd.

Turned TearDown into a real guard that captures and chdirs on
construction and restores on Drop. Since set_current_dir is
process-wide state and tests run concurrently by default, added
#[serial] to test_installer (the only call site not already inside a
#[serial] macro-generated fn) so no two TearDown-holding tests can
race on cwd; serial_test's plain #[serial] shares one unnamed lock
across the whole binary, so this is sufficient.

0 files changed, 0 insertions, 0 deletions