meson: Disable LTO for tests

`-flto=auto` together with `-Wl,-wrap` causes link errors at least in
certain compilers (e.g. GCC 10.2.0). Since this is reoccurring issue
(internet search shows similar problems with GCC 4.6 a decade ago) let's
disable LTO for tests even if it's disabled elsewhere.

Fixes: xorg/xserver#1116
Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
4 jobs for meson-autotools-compatibility in 5 minutes and 49 seconds
Status Job ID Name Coverage
  Docker Image
passed debian-buster #8703236

00:00:25

 
  Build And Test
passed autotools #8703237

00:05:23

passed meson #8703238

00:02:41

passed meson-noglamor #8703239

00:01:44