tests/float: Improve formatfloat.c test coverage using Python.

tests/float/string_format_modulo.py

@@ -42,3 +42,5 @@ print(('%.40g' % 1e-1)[:2])
 print(('%.40g' % 1e-2)[:2])
 print(('%.40g' % 1e-3)[:2])
 print(('%.40g' % 1e-4)[:2])
+
+print("%.0g" % 1) # 0 precision 'g'

tests/float/string_format_modulo3.py

+# uPy and CPython outputs differ for the following
+print("%.1g" % -9.9) # round up 'g' with '-' sign
+print("%.1e" % 9.99) # round up with positive exponent
+print("%.1e" % 0.999) # round up with negative exponent

tests/float/string_format_modulo3.py.exp

+-10
+1.00e+01
+1.00e-00