Testlib: generate_expected_output.py: Run in C locale

Otherwise, float numbers are formatted using a decimal comma
in German, causing huge diffs and failures.

Change-Id: Icd85a293d0564cac6be244eb0793611920d0c89c
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/testlib/selftests/generate_expected_output.py

@@ -109,6 +109,7 @@ if isWindows:
     exit()
 
 tests = sys.argv[1:]
+os.environ['LC_ALL'] = 'C'
 if len(tests) == 0:
    tests = subdirs()
 print("Generating " + str(len(tests)) + " test results for: " + qtver + " in: " + rootPath)