Add fpa regression test for fp.to_real denormal encoding fix

Adds src/test/fpa.cpp with test_fp_to_real_denormal() exercising the
bug reported in the issue: denormal floating-point values in
(_ FloatingPoint 2 24) were getting wrong fp.to_real values because
mk_to_real was not subtracting the normalization shift lz from the
exponent.

Tests verify:
- The specific denormal from the bug report is NOT > 1.0
- Two other denormals have correct real values (0.5 and 0.125)
- A normal value is correctly identified as > 1.0

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/test/CMakeLists.txt

@@ -58,6 +58,7 @@ add_executable(test-z3
   f2n.cpp
   finite_set.cpp
   finite_set_rewriter.cpp
+  fpa.cpp
   factor_rewriter.cpp
   finder.cpp
   fixed_bit_vector.cpp