FortranCInterface: Fix cross-compiling Linux to MinGW (#14358)

After building the test binary tell find_program to search for it with
the ${CMAKE_EXECUTABLE_SUFFIX} so that the .exe can be found.  Since
find_program is normally used to locate host tools while cross-compiling
it needs this hint to find the target binary.

Suggested-by: Denis Barbier <bouzim@gmail.com>
Merge topic 'FortranCInterface-cross-compile' into next

d7a65a2 FortranCInterface: Fix cross-compiling Linux to MinGW (#14358)