Google have already added a subset of dEQP into Android CTS test,
and we believe this part of dEQP have higher priority than the rest
of dEQP test cases.
The case list is stored at some xml files. Such as:
com.drawelements.deqp.gles3.xml.
It's git repo lives in the Android tree at
[https://android.googlesource.com/platform/external/

 \
deqp/+/master/android/cts/com.drawelements.deqp.gles3.xml]

This patch is based on Chad's patch(Add test profile for external
dEQP-GLES3 tests), and add an option to run the subset of dEQP-GLES3.
The only differnce for the running method is: you need to set the
environment variable PIGLIT_DEQP_MUSTPASS or the piglit.conf
option deqp-gles3.mustpasslist,then it will run the subset of dEQP
follow the test case list. If not set, it will still run the whole
dEQP.

Tested on Intel HSW. There are 45866 test cases for the whole dEQP
and 37354 test cases for the subset of dEQP using Android CTS test
case list.

V2:
1.Update the link of get test case list file.
2.Using xml.etree.cElementTree module instead of re module.
3.Fix some Piglit style issue

V3:
1.Update the output issue.
2.Fix some Piglit style issue.

V4:
1.Fix some Piglit style issue.

V5:
1.Fix some Piglit style issue.

v6 (Dylan):
- Remove trailing whitespace

Signed-off-by: Wang Shuo <shuo.wang@intel.com>
Reviewed-by: Dylan Baker <dylanx.c.baker@intel.com>