Fixed BufferGLTest on ES2 non-deprecated build.

Combinatorial explosion of build problems.
MagnumFont: fixed compilation with Clang.
TgaImporter: fix test on ES2.
Fixed constexpr issues in ColorTest under GCC 4.6.
TgaImporter: ColorFormat::Luminance is available only on ES2.
Trade: explicitly link tests to CORRADE_PLUGINMANAGER_LIBRARIES.

Compilation of tests in static build fails with missing
reference to dlclose().