CI/travis: add brotli and zstd to the libssh2 build

... to make sure such tests are run with valgrind. Suppress the zstd
valgrind warnings we get with version 1.3.3 on Ubuntu 18.04 (for debug
and non-debug builds).

Closes #6105

.travis.yml

@@ -59,6 +59,8 @@ jobs:
         - *common_packages
         - krb5-user
         - libssh2-1-dev
+        - libbrotli-dev
+        - libzstd-dev
   - env:
     - T=normal C=--with-libssh
     # Avoid bionic, its pre-release libssh version triggers deprecation warnings.

tests/valgrind.supp

@@ -1,3 +1,35 @@
+{
+   zstd_decompression-1.3.3-on-Ubuntu-18.04
+   Memcheck:Cond
+   fun:ZSTD_decompressStream
+   fun:zstd_unencode_write
+   fun:Curl_unencode_write
+   fun:readwrite_data
+   fun:Curl_readwrite
+   fun:multi_runsingle
+   fun:curl_multi_perform
+   fun:easy_transfer
+   fun:easy_perform
+   fun:curl_easy_perform
+   fun:serial_transfers
+   fun:run_all_transfers
+   fun:operate
+   fun:main
+}
+
+{
+   zstd_decompression-1.3.3-on-Ubuntu-18.04-nondebug
+   Memcheck:Cond
+   fun:ZSTD_decompressStream
+   fun:zstd_unencode_write
+   fun:Curl_readwrite
+   fun:multi_runsingle
+   fun:curl_multi_perform
+   fun:curl_easy_perform
+   fun:operate
+   fun:main
+}
+
 {
    libidn-idna_to_ascii-error
    Memcheck:Addr4