pytests: Test gpu regulator voltage

This also adds helper that makes testing other regulators simple.

pytests/test_regulators.py

+import re
+
+
+def check_voltage(regulator, voltage):
+    r = re.compile(r" +{} ".format(regulator))
+    for line in open('/sys/kernel/debug/regulator/regulator_summary').readlines():
+        if r.match(line):
+            val = line.split()[5]
+            assert(val == "{}mV".format(voltage))
+            return
+    assert(False)
+
+
+# This is true until the GPU does DVFS
+def test_gpu_regulator():
+    check_voltage("buck5", 900)