Make test-wpt supports --release argument

[acgtyrant]

Fixes #5056

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @kmcallister (or someone else) soon.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4114

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[Manishearth]

This doesn't work :/

tests/wpt/run.sh: line 9: [: ==: unary operator expected

[bors-servo]

saw approval from Manishearth
at https://github.com/acgtyrant/servo/commit/b80593c09e9ae37d9d13b4e8c988cf1e8da7efcb

[bors-servo]

merging acgtyrant/servo/patch-1 = b80593c into auto

[bors-servo]

acgtyrant/servo/patch-1 = b80593c merged ok, testing candidate = 3fc2478

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/866
success: http://build.servo.org/builders/linux2/builds/864
success: http://build.servo.org/builders/android/builds/900
success: http://build.servo.org/builders/mac1/builds/1081
success: http://build.servo.org/builders/mac2/builds/1081
success: http://build.servo.org/builders/gonk/builds/52

[bors-servo]

fast-forwarding master to auto = 3fc2478