Autotools' serial test harness uses TESTS_ENVIRONMENT, and doesn't
support AM_TESTS_ENVIRONMENT [1]. The latest version of glib-tap.mk in
GLib targets the newer and default parallel test harness, and removing
the "serial-tests" Automake option breaks the graphical test:
  make[5]: Entering directory
    '/home/rishi/devel/gnome-photos/git/gnome-photos/builddir/tests'
  ERROR: basic.py - missing test plan
  tap-driver.sh: internal error getting exit status
  tap-driver.sh: fatal: I/O or internal error
  make[5]: *** [Makefile:1207: basic.py.log] Error 1

Fortunately, there was no significant breakage because the existing
test doesn't use APIs like g_test_build_filename, g_test_get_filename,
and g_test_get_dir, which would have been affected by the missing
G_TEST_BUILDDIR and G_TEST_SRCDIR environment variables.

Since the Autotools build is deprecated, it's easier to stick to the
serial harness instead of trying to fix the graphical test ...