tests: Add test for pyboard SPI in slave mode, recv with no master.

See PR #1414.

tests/pyb/spi.py

@@ -18,6 +18,12 @@ print(spi)
 
 spi.init(SPI.SLAVE, phase=1)
 print(spi)
+try:
+    # need to flush input before we get an error (error is what we want to test)
+    for i in range(10):
+        spi.recv(1, timeout=100)
+except OSError:
+    print("OSError")
 
 spi.init(SPI.MASTER)
 spi.send(1, timeout=100)

tests/pyb/spi.py.exp

@@ -9,5 +9,6 @@ ValueError Z
 SPI(1)
 SPI(1, SPI.MASTER, baudrate=328125, prescaler=256, polarity=1, phase=0, bits=8)
 SPI(1, SPI.SLAVE, polarity=1, phase=1, bits=8)
+OSError
 b'\xff'
 b'\xff'