gdb/testsuite: rename source file gdb.debuginfod/main.c

gdb/testsuite: rename source file gdb.debuginfod/main.c

The test gdb.debuginfod/fetch_src_and_symbols.exp uses a source file
named main.c.  I can't see any particular reason why the file is named
as such.

Usually test source files are named after the test script.

This commit just renames the source file inline with the test script,
and updates the call to standard_testfile (removing the reference to
main.c).

There's no particular reason for this change other than seeing the
file named main.c made me thing that the source file must be shared
with some other test (it isn't).

There should be no change in what is tested after this commit.