Implementing autopkgtest in SpFFT

This merge request adds Autopkgtest support:

• Updated d/control to define a new package libspfft1-tests for test binaries.
• Updated d/rules to include -DSPFFT_BUILD_TESTS=ON, -DSPFFT_BUNDLED_LIBS=OFF.
• Added the necessary build dependencies such as libgtest-dev, libcli11-dev, nlohmann-json3-dev.
• Added d/libspfft1-tests.install for the installation of tests executables.
• Added d/tests/control, d/tests/run-testdir to handle the execution of tests.
• Set RPATH for the test library.

Tested on GFX90A.