Disable flaky multithreaded tests

The following multithreaded tests are flaky in CQ and are being
suppressed:

    VulkanMultithreadingTest.MultiContextDrawSmallDescriptorPools/ES3_Vulkan
    MultithreadingTestES3.MultithreadFenceDraw/ES3_Vulkan_SwiftShader

Bug: angleproject:5418
Bug: angleproject:6633
Change-Id: I170085064597165972b7206d98130917bb46fcaf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3251684
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>

src/tests/gl_tests/MultithreadingTest.cpp

@@ -725,7 +725,7 @@ void MultithreadingTestES3::mainThreadDraw(bool useDraw)
 TEST_P(MultithreadingTestES3, MultithreadFenceDraw)
 {
     // http://anglebug.com/5418
-    ANGLE_SKIP_TEST_IF(IsLinux() && IsIntel() && IsVulkan());
+    ANGLE_SKIP_TEST_IF(IsLinux() && IsVulkan() && (IsIntel() || IsSwiftshaderDevice()));
 
     // Have the secondary thread use glDrawArrays()
     mainThreadDraw(true);

src/tests/gl_tests/VulkanMultithreadingTest.cpp

@@ -126,6 +126,8 @@ class VulkanMultithreadingTest : public ANGLETest
 // descriptor pools.
 TEST_P(VulkanMultithreadingTest, MultiContextDrawSmallDescriptorPools)
 {
+    // TODO(http://anglebug.com/6633: Flaky on linux.
+    ANGLE_SKIP_TEST_IF(IsLinux());
     ANGLE_SKIP_TEST_IF(!platformSupportsMultithreading());
     // Verify the extension is enabled.
     ASSERT_TRUE(IsGLExtensionEnabled(kExtensionName));