[gdb/testsuite] Fix duplicates in gdb.fortran/huge.exp

With test-case gdb.fortran/huge.exp, on a system without fortran compiler, I
ran into a number of duplicates:
...
Running /home/vries/gdb/src/gdb/testsuite/gdb.fortran/huge.exp ...
gdb compile failed, default_target_compile: Can't find gfortran.
UNTESTED: gdb.fortran/huge.exp: huge.exp
  ...
gdb compile failed, default_target_compile: Can't find gfortran.
UNTESTED: gdb.fortran/huge.exp: huge.exp
DUPLICATE: gdb.fortran/huge.exp: huge.exp
UNSUPPORTED: gdb.fortran/huge.exp: require failed: expr $compilation_succeeded
...

Fix this by wrapping the compile in a with_test_prefix, getting us instead:
...
gdb compile failed, default_target_compile: Can't find gfortran.
UNTESTED: gdb.fortran/huge.exp: CRASH_GDB=2097152a: huge.exp
  ...
gdb compile failed, default_target_compile: Can't find gfortran.
UNTESTED: gdb.fortran/huge.exp: CRASH_GDB=16: huge.exp
UNSUPPORTED: gdb.fortran/huge.exp: require failed: expr $compilation_succeeded
...

Tested on x86_64-linux.