InputPlatformProbe.x11_platform_found_and_used_when_display_connection_works breaks with old input drivers present

When there is a bump in the graphics platform ABI but no simultaneous bump in the input side we end up having more usable X11 platforms than expected. Because of the bump we get two x11 server modules:

$ ls lib/server-modules/
graphics-android.so.7 graphics-dummy.so graphics-mesa-kms.so.8 input-evdev.so.4 input-stub.so server-mesa-x11.so.8
graphics-android.so.8 graphics-mesa-kms.so.7 graphics-throw.so input-evdev.so.5 server-mesa-x11.so.7

And as the log shows both are supported as input platforms (but only one would work as a graphics platform):

$ bin/mir_unit_tests --gtest_filter=InputPlatformProbe*
MIR_CLIENT_PLATFORM_PATH=bin/../lib/client-modules/
MIR_SERVER_PLATFORM_PATH=bin/../lib/server-modules/
LD_LIBRARY_PATH=bin/../lib
exec=bin/mir_unit_tests.bin
Running main() from main.cpp
Note: Google Test filter = InputPlatformProbe*
[==========] Running 4 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 4 tests from InputPlatformProbe
[ RUN ] InputPlatformProbe.stub_platform_not_picked_up_by_default
[2016-02-08 10:25:31.446023] mirserver: Selected input driver: mir:evdev-input (version: 0.20.0)
[ OK ] InputPlatformProbe.stub_platform_not_picked_up_by_default (34 ms)
[ RUN ] InputPlatformProbe.x11_platform_found_and_used_when_display_connection_works
[2016-02-08 10:25:31.460373] mirserver: Selected input driver: mir:x11-input (version: 0.20.0)
[2016-02-08 10:25:31.462505] mirserver: Selected input driver: x11-input (version: 0.19.0)
[2016-02-08 10:25:31.488286] mirserver: Selected input driver: mir:evdev-input (version: 0.20.0)
/home/alan/display_server/development-branch/tests/unit-tests/input/test_input_platform_probing.cpp:149: Failure
Value of: platforms
Expected: has 2 elements and there exists some permutation of elements such that:
 - element #0 is a or derived from a N3mir5input5evdev8PlatformE, and
 - element #1 is a or derived from a N3mir5input1X14XInputPlatformE
  Actual: { 24-byte object <F0-8F 46-04 00-00 00-00 E0-8F 46-04 00-00 00-00 00-D4 43-04 00-00 00-00>, 24-byte object <60-94 46-04 00-00 00-00 50-94 46-04 00-00 00-00 80-DA 43-04 00-00 00-00>, 24-byte object <A0-D5 43-04 00-00 00-00 90-D5 43-04 00-00 00-00 50-DA 45-04 00-00 00-00> }, which has 3 elements
[ FAILED ] InputPlatformProbe.x11_platform_found_and_used_when_display_connection_works (42 ms)
[ RUN ] InputPlatformProbe.x11_input_platform_not_used_when_vt_specified
[2016-02-08 10:25:31.524523] mirserver: Selected input driver: mir:evdev-input (version: 0.20.0)
[ OK ] InputPlatformProbe.x11_input_platform_not_used_when_vt_specified (36 ms)
[ RUN ] InputPlatformProbe.allows_forcing_stub_input_platform
[2016-02-08 10:25:31.530326] mirserver: Selected input driver: mir:stub-input (version: 0.20.0)
[ OK ] InputPlatformProbe.allows_forcing_stub_input_platform (5 ms)
[----------] 4 tests from InputPlatformProbe (117 ms total)

[----------] Global test environment tear-down
[==========] 4 tests from 1 test case ran. (118 ms total)
[ PASSED ] 3 tests.
[ FAILED ] 1 test, listed below:
[ FAILED ] InputPlatformProbe.x11_platform_found_and_used_when_display_connection_works

 1 FAILED TEST

If we have a release with a bump like this, users upgrading might end up in a problematic configuration.