valgrind: Add a suppression for a hunspell memory leak during the SpellCheckStrings_EN_US test

tools/valgrind/memcheck/suppressions.txt

 # There are three kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 979 matching lines @@
   fun:_ZN47SpellCheckTest_GetAutoCorrectionWord_EN_US_Test8TestBodyEv
   fun:_ZN7testing4Test3RunEv
   fun:_ZN7testing8internal12TestInfoImpl3RunEv
   fun:_ZN7testing8internal12TestInfoImpl7RunTestEPNS_8TestInfoE
   fun:_ZN7testing8TestCase3RunEv
   fun:_ZN7testing8TestCase11RunTestCaseEPS0_
   fun:_ZN7testing8internal12UnitTestImpl11RunAllTestsEv
   fun:_ZN7testing8UnitTest3RunEv
   fun:main
 }
+{
+  bug_18254b
+  Memcheck:Leak
+  fun:_Znaj
+  fun:_ZN8AffixMgr10parse_fileEv
+  fun:_ZN8AffixMgrC1EPN8hunspell11BDictReaderEPP7HashMgrPi
+  fun:_ZN8HunspellC1EPKhj
+  fun:_ZN12SpellChecker10InitializeEv
+  fun:_ZN12SpellChecker14SpellCheckWordEPKwiPiS2_PSt6vectorISbIwSt11char_traitsIwESaIwEESaIS7_EE
+  fun:_ZN43SpellCheckTest_SpellCheckStrings_EN_US_Test8TestBodyEv
+  fun:_ZN7testing4Test3RunEv
+  fun:_ZN7testing8internal12TestInfoImpl3RunEv
+  fun:_ZN7testing8internal12TestInfoImpl7RunTestEPNS_8TestInfoE
+  fun:_ZN7testing8TestCase3RunEv
+  fun:_ZN7testing8TestCase11RunTestCaseEPS0_
+  fun:_ZN7testing8internal12UnitTestImpl11RunAllTestsEv
+  fun:_ZN7testing8UnitTest3RunEv
+  fun:main
+}