Clear message log at the beginning of async debug tests.

Bug: 22921269
Change-Id: Ia16bc7a0be625f907c1fd7b9aa6de8222e1e5e2c
diff --git a/modules/gles31/functional/es31fDebugTests.cpp b/modules/gles31/functional/es31fDebugTests.cpp
index e8acd2c..ef0cdab 100644
--- a/modules/gles31/functional/es31fDebugTests.cpp
+++ b/modules/gles31/functional/es31fDebugTests.cpp
@@ -1434,6 +1434,13 @@
 	const int				maxWait		= 10000; // ms
 	const int				warnWait	= 100;
 
+	// Clear log from earlier messages
+	{
+		GLint numMessages = 0;
+		gl.getIntegerv(GL_DEBUG_LOGGED_MESSAGES, &numMessages);
+		gl.getDebugMessageLog(numMessages, 0, DE_NULL, DE_NULL, DE_NULL, DE_NULL, DE_NULL, DE_NULL);
+	}
+
 	gl.enable(GL_DEBUG_OUTPUT);
 	gl.enable(GL_DEBUG_OUTPUT_SYNCHRONOUS);
 	gl.debugMessageControl(GL_DONT_CARE, GL_DONT_CARE, GL_DONT_CARE, 0, DE_NULL, false);