tests: Use G_TEST_OPTION_ISOLATE_DIRS in gdbus-peer-object-manager

This might fix a recent test failure: https://gitlab.gnome.org/GNOME/glib/-/jobs/1929015. Unfortunately there’s not much debug information in the logs to go on, and I can’t reproduce it locally. All I have is:

192/272 glib:gio / gdbus-peer-object-manager    FAIL     0.43 s (killed by signal 11 SIGSEGV)

--- command ---
GIO_MODULE_DIR='' G_TEST_BUILDDIR='/builds/GNOME/glib/_build/gio/tests' G_TEST_SRCDIR='/builds/GNOME/glib/gio/tests' /builds/GNOME/glib/_build/gio/tests/gdbus-peer-object-manager
--- stdout ---
\# random seed: R02Seee9b7325ecd7c19249a3412397aed9b
\# Start of gdbus tests
\# Start of peer-object-manager tests

Signed-off-by: Philip Withnall pwithnall@endlessos.org

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