Reset serialize_precision to stabilize tests

Reset serialize_precision to stabilize tests

With serialize_precision = 100 the following
failures were generated:

1) ApiFormatPhpTest::testGeneralEncoding with data set #7
(array(1.0E+42), 'a:1:{i:0;d:1.0E+42;}', array(1))
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-a:1:{i:0;d:1.0E+42;}
+a:1:{i:0;d:1000000000000000044885712678075916785549312;}

/usr/home/saper/public_html/ybabel/tests/phpunit/includes/api/format/ApiFormatTestBase.php:61
/usr/home/saper/public_html/ybabel/tests/phpunit/MediaWikiTestCase.php:137

2) ApiFormatPhpTest::testGeneralEncoding with data set #30
(array(1.0E+42), 'a:1:{i:0;d:1.0E+42;}', array(2))
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-a:1:{i:0;d:1.0E+42;}
+a:1:{i:0;d:1000000000000000044885712678075916785549312;}

/usr/home/saper/public_html/ybabel/tests/phpunit/includes/api/format/ApiFormatTestBase.php:61
/usr/home/saper/public_html/ybabel/tests/phpunit/MediaWikiTestCase.php:137

Bug: T116683
Change-Id: I5e760b4632d2ab326954fe5769476e143c376d7c