tests: add missing config system tests

Add basic tests for the behavior of the west.configuration APIs for
ConfigFile.SYSTEM, which were missing since we couldn't patch out the
location to somewhere safe.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

tests/test_config.py

@@ -145,6 +145,41 @@ def test_config_local():
     assert 'pytest' not in glb
     assert lcl['pytest']['local2'] == 'foo2'
 
+@patch('west.configuration._location', new=tstloc)
+def test_config_system():
+    # Basic test of system-level configuration.
+    #
+    # Since we use tstloc(), we can't call cmd('config ...') here.
+
+    config.update_config('pytest', 'key', 'val', configfile=SYSTEM)
+    assert cfg(f=ALL)['pytest']['key'] == 'val'
+    assert cfg(f=SYSTEM)['pytest']['key'] == 'val'
+    assert 'pytest' not in cfg(f=GLOBAL)
+    assert 'pytest' not in cfg(f=LOCAL)
+
+    config.update_config('pytest', 'key', 'val2', configfile=SYSTEM)
+    assert cfg(f=SYSTEM)['pytest']['key'] == 'val2'
+
+@patch('west.configuration._location', new=tstloc)
+def test_config_system_precedence():
+    # Test precedence rules, including system level.
+    #
+    # Since we use tstloc(), we can't call cmd('config ...') here.
+    config.update_config('pytest', 'key', 'sys', configfile=SYSTEM)
+    assert cfg(f=SYSTEM)['pytest']['key'] == 'sys'
+    assert cfg(f=ALL)['pytest']['key'] == 'sys'
+
+    config.update_config('pytest', 'key', 'glb', configfile=GLOBAL)
+    assert cfg(f=SYSTEM)['pytest']['key'] == 'sys'
+    assert cfg(f=GLOBAL)['pytest']['key'] == 'glb'
+    assert cfg(f=ALL)['pytest']['key'] == 'glb'
+
+    config.update_config('pytest', 'key', 'lcl', configfile=LOCAL)
+    assert cfg(f=SYSTEM)['pytest']['key'] == 'sys'
+    assert cfg(f=GLOBAL)['pytest']['key'] == 'glb'
+    assert cfg(f=LOCAL)['pytest']['key'] == 'lcl'
+    assert cfg(f=ALL)['pytest']['key'] == 'lcl'
+
 @patch('west.configuration._location', new=tstloc)
 def test_system_creation():
     # Test that the system file -- and just that file -- is created on