[version-test] Work around bugs in v0.7.3 dict builder

Before calling a dictionary good, make sure that it can compress an
input. If v0.7.3 rejects v0.7.3's dictionary, fall back to the v1.0
dictionary. This is not the job of the verison test to test it, because
we cannot fix this code.

tests/test-zstd-versions.py

@@ -85,6 +85,18 @@ def get_git_tags():
     return tags
 
 
+def dict_ok(tag, dict_name, sample):
+    if not os.path.isfile(dict_name):
+        return False
+    try:
+        cmd = ['./zstd.' + tag, '-D', dict_name]
+        with open(sample, "rb") as i:
+            subprocess.check_call(cmd, stdin=i, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        return True
+    except:
+        return False
+
+
 def create_dict(tag, dict_source_path, fallback_tag=None):
     dict_name = 'dict.' + tag
     if not os.path.isfile(dict_name):
@@ -96,7 +108,7 @@ def create_dict(tag, dict_source_path, fallback_tag=None):
             result = execute('./dictBuilder.' + tag + ' ' + ' '.join(files) + ' -o ' + dict_name, print_output=False, param_shell=True)
         else:
             result = execute('./zstd.' + tag + ' -f --train ' + ' '.join(files) + ' -o ' + dict_name, print_output=False, param_shell=True)
-        if result == 0 and os.path.isfile(dict_name):
+        if result == 0 and dict_ok(tag, dict_name, files[0]):
             print(dict_name + ' created')
         elif fallback_tag is not None:
             fallback_dict_name = 'dict.' + fallback_tag