support relative compiler filenames in rdmd_test

[marler8997]

No description provided.

[dlang-bot]

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[marler8997]

Can someone restart travis CI? Looks like a networking error.

######################################################################## 100.0%
Downloading gdmd https://raw.githubusercontent.com/D-Programming-GDC/GDMD/130f552ca43a77ee5c638fcc5a106f41dac607b9/dmd-script
######################################################################## 100.0%
curl: (22) The requested URL returned error: 502 Bad Gateway
Downloading and unpacking http://code.dlang.org/files/dub--linux-x86_64.tar.gz
curl: (22) The requested URL returned error: 502 Bad Gateway
tar (child): /home/travis/dlang/.installer_tmp_z6pwsu/LJnzv1/dub.tar.gz: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now