libssh2: skip the testsuite when docker is not installed (#26962)

The build-time testsuite which would be run when building
with tests needs docker. Check that it exists before
attempting to execute the tests.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

var/spack/repos/builtin/packages/libssh2/package.py

@@ -32,3 +32,8 @@ def darwin_fix(self):
         # The shared library is not installed correctly on Darwin; fix this
         if self.spec.satisfies('platform=darwin'):
             fix_darwin_install_name(self.prefix.lib)
+
+    def check(self):
+        # Docker is required to run tests
+        if which('docker'):
+            make('test')