tests: add certificate check against letsencrypt.org

In order to prevent regressions like #9283, lets add runtime check
against letsencrypt.org.

References: https://github.com/openwrt/openwrt/issues/9283
References: https://github.com/wolfSSL/wolfssl/issues/4879
Signed-off-by: Petr Štetiar <ynezz@true.cz>

tests/cram/test-san_uclient-fetch.t

@@ -65,6 +65,8 @@ check that SSL works:
 
   $ uc -q -O /dev/null 'https://www.openwrt.org'
 
+  $ uc -q -O /dev/null 'https://letsencrypt.org'
+
   $ uc -O /dev/null 'https://downloads.openwrt.org/does-not-exist' 2>&1 | grep error
   HTTP error 404
 

tests/cram/test_uclient-fetch.t

@@ -65,6 +65,8 @@ check that SSL works:
 
   $ uc -q -O /dev/null 'https://www.openwrt.org'
 
+  $ uc -q -O /dev/null 'https://letsencrypt.org'
+
   $ uc -O /dev/null 'https://downloads.openwrt.org/does-not-exist' 2>&1 | grep error
   HTTP error 404