Allow reftests to be run directly w/ relative path

[mttr]

Fixes #3521

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3234

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[SimonSapin]

r+

[bors-servo]

saw approval from SimonSapin
at mttr@3149eae

[bors-servo]

merging mttr/servo/reftests_with_relative_dir = 3149eae into auto

[bors-servo]

mttr/servo/reftests_with_relative_dir = 3149eae merged ok, testing candidate = dfa9e1a

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/25
success: http://build.servo.org/builders/linux2/builds/25
success: http://build.servo.org/builders/android/builds/92
success: http://build.servo.org/builders/mac1/builds/242
success: http://build.servo.org/builders/mac2/builds/242

[bors-servo]

fast-forwarding master to auto = dfa9e1a