Check stepUp() and stepDown() behavior for type=datetime-local.

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

Invalid value
PASS stepUp("", null, null) is "1970-01-01T00:01"
PASS stepDown("", null, null) is "1969-12-31T23:59"
Non-number arguments
PASS stepUp("2010-02-10T20:13", null, null, "0") is "2010-02-10T20:13"
PASS stepDown("2010-02-10T20:13", null, null, "0") is "2010-02-10T20:13"
PASS stepUp("2010-02-10T20:13", null, null, "foo") is "2010-02-10T20:13"
PASS stepDown("2010-02-10T20:13", null, null, "foo") is "2010-02-10T20:13"
PASS stepUp("2010-02-10T20:13", null, null, null) is "2010-02-10T20:13"
PASS stepDown("2010-02-10T20:13", null, null, null) is "2010-02-10T20:13"
Normal cases
PASS stepUp("2010-02-10T20:13", null, null) is "2010-02-10T20:14"
PASS stepDown("2010-02-10T20:13", null, null) is "2010-02-10T20:12"
PASS stepUp("2010-02-10T20:13", null, null, 10) is "2010-02-10T20:23"
PASS stepDown("2010-02-10T20:13", null, null, 11) is "2010-02-10T20:02"
PASS stepUp("1970-01-01T20:13", "4", null, 2) is "1970-01-01T20:13:08"
PASS stepDown("1970-01-01T20:13", "4", null, 3) is "1970-01-01T20:12:48"
Step=any
PASS stepUp("2010-02-10T20:13", "any", null) threw exception InvalidStateError: Failed to execute 'stepUp' on 'HTMLInputElement': This form element does not have an allowed value step..
PASS stepDown("2010-02-10T20:13", "any", null) threw exception InvalidStateError: Failed to execute 'stepDown' on 'HTMLInputElement': This form element does not have an allowed value step..
Overflow/underflow
PASS stepUp("2010-02-10T20:13", "3.40282346e+38", null) is "1970-01-01T00:00"
PASS stepDown("2010-02-10T20:13", "3.40282346e+38", null) is "1970-01-01T00:00"
PASS stepUp("2010-02-10T20:13", "1", "2010-02-10T20:13") is "2010-02-10T20:13"
PASS stepDown("2010-02-10T20:13", "1", "2010-02-10T20:13") is "2010-02-10T20:13"
PASS successfullyParsed is true

TEST COMPLETE