Fix python sys.platform check for Linux (elastic#15727)

On Linux, sys.platform doesn’t contain the major version anymore. It is now always ‘linux’, instead of ‘linux2’ or ‘linux3’ depending on the Linux version used to build Python. Replace sys.platform == ‘linux2’ with sys.platform.startswith(‘linux’), or directly sys.platform == ‘linux’ if you don’t need to support older Python versions.

Ref: https://docs.python.org/3.3/whatsnew/3.3.html#porting-python-code

x-pack/auditbeat/tests/system/test_metricsets.py

@@ -24,7 +24,7 @@ def test_metricset_host(self):
 
         self.check_metricset("system", "host", COMMON_FIELDS + fields)
 
-    @unittest.skipUnless(sys.platform == "linux2", "Only implemented for Linux")
+    @unittest.skipUnless(sys.platform.startswith('linux'), "Only implemented for Linux")
     @unittest.skipIf(sys.byteorder != "little", "Test only implemented for little-endian systems")
     def test_metricset_login(self):
         """
@@ -43,7 +43,7 @@ def test_metricset_login(self):
         self.check_metricset("system", "login", COMMON_FIELDS + fields, config, warnings_allowed=True)
 
     @unittest.skipIf(sys.platform == "win32", "Not implemented for Windows")
-    @unittest.skipIf(sys.platform == "linux2" and not (os.path.isdir("/var/lib/dpkg") or os.path.isdir("/var/lib/rpm")),
+    @unittest.skipIf(sys.platform.startswith('linux') and not (os.path.isdir("/var/lib/dpkg") or os.path.isdir("/var/lib/rpm")),
                      "Only implemented for dpkg and rpm")
     def test_metricset_package(self):
         """
@@ -74,7 +74,7 @@ def test_metricset_process(self):
         self.check_metricset("system", "process", COMMON_FIELDS + fields, {"process.hash.max_file_size": 1},
                              errors_allowed=True, warnings_allowed=True)
 
-    @unittest.skipUnless(sys.platform == "linux2", "Only implemented for Linux")
+    @unittest.skipUnless(sys.platform.startswith('linux'), "Only implemented for Linux")
     def test_metricset_user(self):
         """
         user metricset collects information about users on a server.