[perf-training] Check extension in findFilesWithExtension

[Amir]

Summary

findFilesWithExtension helper checks for endswith(extension) instead of
exactly matching the file extension. This causes it to match unrelated files,
for example, .profdata files while matching .fdata files:

http://157.230.108.44:8011/#/builders/56/builds/247

Merging data from /worker/worker/bolt-x86_64-ubuntu-clang-bolt-gcc/build/tools/clang/prof.fdata.1124569.fdata...
Merging data from /worker/worker/bolt-x86_64-ubuntu-clang-bolt-gcc/build/tools/clang/test/Frontend/Output/optimization-remark-with-hotness-new-pm.c.tmp.profdata...

[phosek]

LGTM

clang/utils/perf-training/perf-helper.py

 import tempfile
 
 test_env = { 'PATH'    : os.environ['PATH'] }
 
 def findFilesWithExtension(path, extension):
   filenames = []
   for root, dirs, files in os.walk(path): 
     for filename in files:
-      if filename.endswith(extension):
+      if os.path.splitext(filename)[1] == extension:
         filenames.append(os.path.join(root, filename))
   return filenames
 
 def clean(args):
   if len(args) != 2:
     print('Usage: %s clean <path> <extension>\n' % __file__ +
       '\tRemoves all files with extension from <path>.')
     return 1