tests/libqtest: Print error instead of aborting when env variable is missing

When you currently try to run a test directly from the command line
without setting the QTEST_QEMU_BINARY environment variable first,
you are presented with an unhelpful assertion message like this:

 ERROR:tests/libqtest.c:163:qtest_init_without_qmp_handshake:
 assertion failed: (qemu_binary != NULL)
 Aborted (core dumped)

Let's replace the assert() with a more user friendly error message
instead.
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

tests/libqtest.c

@@ -160,7 +160,10 @@ QTestState *qtest_init_without_qmp_handshake(const char *extra_args)
     const char *qemu_binary;
 
     qemu_binary = getenv("QTEST_QEMU_BINARY");
-    g_assert(qemu_binary != NULL);
+    if (!qemu_binary) {
+        fprintf(stderr, "Environment variable QTEST_QEMU_BINARY required\n");
+        exit(1);
+    }
 
     s = g_malloc(sizeof(*s));