Fortran: check for undeclared variables by forcing implicit none everywhere
C/C++: check for unused parameters and return types
removed unused variables from test cases
ci: do missing return and unused arg check with Github Actions
Also add new tests for the platform-specific and compiler-specific
versioning scheme.
A rough summary is:
1. A bug in how run_tests.py:validate_install checked for files has been
fixed. Earlier it wasn't checking the install directory properly.
2. Shared libraries are no longer installed in common tests, and the
library name/path testing is now done in platform-specific tests.
3. Executables are now always called something?exe in the
installed_files.txt file, and the suffix automatically corrected
depending on the platform.
4. If a test installs a file called 'no-installed-files', the installed
files for that test are not validated. This is required to implement
compiler-specific tests for library names/paths such as MSVC vs MinGW
5. The platform-specific file renaming in run_tests.py has been mostly
removed since it is broken for shared libraries and isn't needed for
static libraries.
6. run_tests.py now reports all missing and extra files. The logic for
finding these has been reworked.