DawnTest: Add function to init test environment.

This function is currently empty and added on its own so that it can be
rolled and a called in Chromium's dawn_end2end_tests_main.cc before
functional changes. Otherwise follow-up DawnTest changes wouldn't work
running dawn_end2end_tests built in Chromium.

BUG=dawn:109

Change-Id: Ieb191c415ee6720cab8330d3c2b05f76d263adfc
Reviewed-on: https://dawn-review.googlesource.com/c/4822
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/tests/DawnTest.cpp

@@ -96,6 +96,9 @@ namespace {
 
 }  // namespace
 
+void InitDawnEnd2EndTestEnvironment(int, char**) {
+}
+
 DawnTest::DawnTest() = default;
 
 DawnTest::~DawnTest() {