add negative float conversions to test constantUnaryConversion.comp

add negative float conversions to test constantUnaryConversion.comp

Add conversions from negative float16_t and float32_t to bool, all
signed integer types (i.e., including those in
GL_EXT_shader_explicit_arithmetic_types), and all float types (from the
same extension) to extend coverage.

Note that converting negative float values to unsigned integers is
undefined behavior. Thus, we exclude them.