meson: only build mesa_st tests when build-tests is true

Since we have an option to turn test building on and off, we should
honor that.

Fixes: 34cb4d0ebc
       ("meson: build tests for gallium mesa state tracker")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/meson.build

@@ -82,7 +82,9 @@ if with_gallium
   subdir('gallium')
   # This has to be here since it requires libgallium, and subdir cannot
   # contain ..
-  subdir('mesa/state_tracker/tests')
+  if with_tests
+    subdir('mesa/state_tracker/tests')
+  endif
 endif
 
 # This must be after at least mesa, glx, and gallium, since libgl will be