Add simple autotests

Two main tests: read the file and compare to a given image (loaded from
png), and write a file out and compare to a previously-written image.

The write test in particular is kind of fragile, but it should help
catch some errors where a code change was not intended to affect the
output of the plugin.

Tests are included for all the writable formats, as well as every
readable format that GIMP can write to (and so I could easily get an
example of).

REVIEW: 115430

tests/CMakeLists.txt

@@ -2,8 +2,6 @@ add_definitions(-DPLUGIN_DIR="${CMAKE_CURRENT_BINARY_DIR}/../src")
 
 include(ECMMarkAsTest)
 
-find_package(Qt5 5.2.0 CONFIG REQUIRED Test Gui)
-
 macro(kimageformats_executable_tests)
     foreach(_testname ${ARGN})
         add_executable(${_testname} ${_testname}.cpp)