Do not build tests when introspection is disabled/missing

Fixes:
| ../libpeas-1.26.0/tests/libpeas/introspection/meson.build:47:0: ERROR: Unknown variable "libpeas_gir".

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
4 jobs for master in 9 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #851026
build peas

00:09:05

passed #851027
build peas-gtk

00:09:19

 
  Deploy
passed #851028
pages

00:00:29

passed pages:deploy

00:00:00