osmesa/test: Clear the stencil bits in the depth test.

Since the depth buffer starts out as a malloc, and we weren't clearing it,
you could get undefined values in your top 8 bits.  This should fix
intermittent failures of the depth test.

(Sadly, valgrind wasn't catching this, presumably because the 32-bit value
there *is* written, just some bits are left undef)

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1243>

src/gallium/targets/osmesa/test-render.cpp

@@ -186,7 +186,7 @@ TEST(OSMesaRenderTest, depth)
    ASSERT_EQ(depth_cpp, 4);
 
    glClearDepth(1.0);
-   glClear(GL_DEPTH_BUFFER_BIT);
+   glClear(GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
    glFinish();
    EXPECT_EQ(depth[w * 0 + 0], 0x00ffffff);
    EXPECT_EQ(depth[w * 0 + 1], 0x00ffffff);