adjust test and assertion to be more flexible

tests/Composer/Test/Fixtures/installer/github-issues-4319.test.skipped

@@ -37,7 +37,7 @@ Your requirements could not be resolved to an installable set of packages.
 
   Problem 1
     - Installation request for a ~1.0 -> satisfiable by a[1.0.0].
-    - a 1.0.0 requires php 5.5 -> your PHP version (5.6.17) overriden by "config.platform.php" version (5.3) does not satisfy that requirement.
+    - a 1.0.0 requires php 5.5 -> your PHP version (%s) overriden by "config.platform.php" version (5.3) does not satisfy that requirement.
 
 --EXPECT--
 

tests/Composer/Test/InstallerTest.php

@@ -252,7 +252,7 @@ class InstallerTest extends TestCase
         $this->assertSame(rtrim($expect), implode("\n", $installationManager->getTrace()));
 
         if ($expectOutput) {
-            $this->assertEquals(rtrim($expectOutput), rtrim($output));
+            $this->assertStringMatchesFormat(rtrim($expectOutput), rtrim($output));
         }
     }