tools run-cts: Add --coverage flag

Displays a per-test coverage viewer in your browser

Change-Id: I0b808bfadf01dab0540143760580cd7ca680e93b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113644
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>

tools/src/git/git.go

@@ -64,6 +64,13 @@ type Git struct {
 
 // New returns a new Git instance
 func New(exe string) (*Git, error) {
+	if exe == "" {
+		g, err := exec.LookPath("git")
+		if err != nil {
+			return nil, fmt.Errorf("failed to find git: %v", err)
+		}
+		exe = g
+	}
 	if _, err := os.Stat(exe); err != nil {
 		return nil, err
 	}