minor symfony#38321 Fix flaky test possiblity on PHP >=7.1 (Valentin)

This PR was merged into the 5.1 branch.

Discussion
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Fix flaky test possiblity on PHP >=7.1

| Q             | A
| ------------- | ---
| Branch?       | 5.1
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix symfony#38320
| License       | MIT
| Doc PR        | not needed

Commits
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f99256f Fix flaky test possiblity on PHP >=7.1

Author: nicolas-grekas

.appveyor.yml

@@ -25,7 +25,7 @@ install:
     - cd ..
     - copy /Y php.ini-development php.ini-min
     - echo memory_limit=-1 >> php.ini-min
-    - echo serialize_precision=14 >> php.ini-min
+    - echo serialize_precision=-1 >> php.ini-min
     - echo max_execution_time=1200 >> php.ini-min
     - echo date.timezone="America/Los_Angeles" >> php.ini-min
     - echo extension_dir=ext >> php.ini-min