gnu: mesa: Disable failing test on i686.

* gnu/packages/gl.scm (mesa)[arguments]: Add phase when building for i686.

gnu/packages/gl.scm

@@ -328,6 +328,15 @@ also known as DXTn or DXTC) for Mesa.")
              (substitute* "src/intel/genxml/gen_pack_header.py"
                (("/usr/bin/env python2") (which "python")))
              #t))
+         ,@(if (string-prefix? "i686" (%current-system))
+               ;; Disable new test from Mesa 19 that fails on i686.  Upstream
+               ;; report: <https://bugs.freedesktop.org/show_bug.cgi?id=110612>.
+               `((add-after 'unpack 'disable-failing-test
+                   (lambda _
+                     (substitute* "src/gallium/tests/unit/meson.build"
+                       (("'u_format_test',") ""))
+                     #t)))
+               '())
          (add-before
            'configure 'fix-dlopen-libnames
            (lambda* (#:key inputs outputs #:allow-other-keys)