Verify that reftest passes valid servo args

[mttr]

Fixes #4101

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3328

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[jdm]

r+

[jdm]

@bors: retry

• servo#3595

[bors-servo]

saw approval from jdm
at mttr@41e50b0

[bors-servo]

merging mttr/servo/reftest_unknown_servo_args = 41e50b0 into auto

[bors-servo]

mttr/servo/reftest_unknown_servo_args = 41e50b0 merged ok, testing candidate = 7badd93

[bors-servo]

some tests failed:
failure: http://build.servo.org/builders/linux2/builds/104

[bors-servo]

saw approval from jdm
at mttr@41e50b0

[bors-servo]

merging mttr/servo/reftest_unknown_servo_args = 41e50b0 into auto

[bors-servo]

mttr/servo/reftest_unknown_servo_args = 41e50b0 merged ok, testing candidate = c682690

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/110
success: http://build.servo.org/builders/linux2/builds/110
success: http://build.servo.org/builders/android/builds/177
success: http://build.servo.org/builders/mac1/builds/327
success: http://build.servo.org/builders/mac2/builds/327

[bors-servo]

fast-forwarding master to auto = c682690