Prevent Rust from constant folding in FPU test

[hydrolarus]

The operations were done at compile time, so the floating point operations didn't end up in the binary - even debug ones!

This commit wraps all the relevant floating point values in a black_box which prevents the compiler from "seeing" the values, so it emits proper FPU instructions in all cases.

fixes #21

[hydrolarus]

With those changes and the FPU disabled in the CPU configuration, this is the result of the tests

lara@gronau:~/Projects/clash-vexriscv$ cabal run clash-vexriscv-sim:unittests
VexRiscv Tests
  Debug builds
    Integration test `long_string` (debug):        OK (4.38s)
    Integration test `many_calls` (debug):         OK (0.78s)
    Integration test `hello_world` (debug):        OK (0.33s)
    Integration test `print_numbers` (debug):      OK (3.42s)
    Integration test `dyn_dispatch` (debug):       OK (0.22s)
    Integration test `ebreak_exception` (debug):   OK (1.28s)
    Integration test `loop_write` (debug):         OK (8.73s)
    Integration test `fpu_test` (debug):           FAIL (21.28s)
      tests/tests.hs:53:
      expected: "This test makes sure the FPU works as expected\n79823i32 as f32 = 79823\n1.3 + 5.3 = 6.6000004\n5.3 - 1.3 = 4\n24.65 * 43.2 = 1064.88\n12.6 / 4.2 = 3.0000002\n"
       but got: "This test makes sure the FPU works as expected\n79823i32 as f32 = 79823\n"
      Use -p '/Integration test `fpu_test` (debug)/' to rerun this test only.
    Integration test `single_write` (debug):       OK (0.63s)
  Release builds
    Integration test `long_string` (release):      OK (0.23s)
    Integration test `many_calls` (release):       OK (0.26s)
    Integration test `hello_world` (release):      OK (0.18s)
    Integration test `print_numbers` (release):    OK (0.36s)
    Integration test `dyn_dispatch` (release):     OK (0.16s)
    Integration test `ebreak_exception` (release): OK (0.16s)
    Integration test `loop_write` (release):       OK (0.80s)
    Integration test `fpu_test` (release):         FAIL (21.91s)
      tests/tests.hs:53:
      expected: "This test makes sure the FPU works as expected\n79823i32 as f32 = 79823\n1.3 + 5.3 = 6.6000004\n5.3 - 1.3 = 4\n24.65 * 43.2 = 1064.88\n12.6 / 4.2 = 3.0000002\n"
       but got: "This test makes sure the FPU works as expected\n79823i32 as f32 = 79823\n"
      Use -p '/Integration test `fpu_test` (release)/' to rerun this test only.
    Integration test `single_write` (release):     OK (0.23s)

2 out of 18 tests failed (65.38s)