test: Make test_gpio_read() independent

This assumes that the GPIO starts as 0 but it does not if
test_gpio_input() ran first and test_gpio_exit_statuses() was skipped.
This can happen when running tests in parallel.

Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_gpio.py b/test/py/tests/test_gpio.py
index fa0af5f..0af186f 100644
--- a/test/py/tests/test_gpio.py
+++ b/test/py/tests/test_gpio.py
@@ -51,6 +51,7 @@
 def test_gpio_read(u_boot_console):
     """Test that gpio read correctly sets the variable to the value of a gpio pin."""
 
+    u_boot_console.run_command('gpio clear 0')
     response = u_boot_console.run_command('gpio read var 0; echo val:$var,rc:$?')
     expected_response = 'val:0,rc:0'
     assert(expected_response in response)