test: Use test scenario for selecting browser

Similar to what cockpit-composer does. This is rather simplistic, but
does fine until we need more complex scenarios.

Also use plain shell in test/run, there are no bashisms.

Closes #291

test/run

@@ -1,4 +1,8 @@
-#! /bin/bash
+#! /bin/sh
+set -eu
+
 # This is the expected entry point for Cockpit CI; will be called without
-# arguments but with an appropriate $TEST_OS
+# arguments but with an appropriate $TEST_OS, and optionally $TEST_SCENARIO
+
+[ -z "${TEST_SCENARIO:-}" ] || export TEST_BROWSER="$TEST_SCENARIO"
 make check