Add a newline at the end of test files

When running in some settings, a C compiler may demand newlines at the
end of each file.  Instead of modifying everywhere that writes out test
files to incorporate newlines in each indivudual string, simply add a
newline when writing it out.

Only add a newline to the end of the file if there isn't one already
there.

An examples of when this is a problem is running with `CC=clang` and
`CFLAGS="--std=c99 -pedantic-errors"` and meson.build contains (for
example) the following:

```
project('myproject', 'c')
executable('myexecutable', 'main.c')
cc = meson.get_compiler('c')
sizeof_int = cc.sizeof('int')

```

sizeof_int will be -1, because the compile failed.  The meson logs
contain the error `testfile.c:7:10: error: no newline at end of file`

mesonbuild/compilers/compilers.py

@@ -805,6 +805,8 @@ class Compiler(HoldableObject, metaclass=abc.ABCMeta):
                                        'testfile.' + self.default_suffix)
                 with open(srcname, 'w', encoding='utf-8') as ofile:
                     ofile.write(code)
+                    if not code.endswith('\n'):
+                        ofile.write('\n')
                 # ccache would result in a cache miss
                 no_ccache = True
                 code_debug = f'Code:\n{code}'