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/main.cpp

@@ -287,4 +287,5 @@ int main(int argc, char ** argv) {
     TST(ho_matcher);
     TST(finite_set);
     TST(finite_set_rewriter);
+    TST(fpa);
 }