[gdb/testsuite] Fix double debug info in gdb.dwarf2/dw2-ref-missing-frame.exp

A mistake slipped in in commit a5ea23036d "[gdb/testsuite] Use function_range
in gdb.dwarf2/dw2-ref-missing-frame.exp".

Before the commit the main file was compiled with debug info, and the two
others not:
...
if {[prepare_for_testing_full "failed to prepare" \
        [list $testfile {} $srcfile {} $srcfuncfile {} \
             $srcmainfile debug]]} {
...

After the commit, all were compiled with debug info, and consequently, there
are two versions of debug info for $srcfuncfile.  This shows up as a FAIL when
running the test-case with target boards readnow and cc-with-debug-names.

Fix this by using prepare_for_testing_full, as before.

Tested on x86_64-linux.

Fixes: a5ea23036d ("[gdb/testsuite] Use function_range in
       gdb.dwarf2/dw2-ref-missing-frame.exp")

gdb/testsuite/gdb.dwarf2/dw2-ref-missing-frame.exp

@@ -29,15 +29,17 @@ lassign [function_range func_loopfb \
 
 set flags \
     [list \
-         "debug" \
          "additional_flags=\"-DFUNC_NOFB_START=$func_nofb_start\"" \
          "additional_flags=\"-DFUNC_NOFB_END=$func_nofb_start + $func_nofb_len\"" \
          "additional_flags=\"-DFUNC_LOOPFB_START=$func_loopfb_start\"" \
          "additional_flags=\"-DFUNC_LOOPFB_END=$func_loopfb_start + $func_loopfb_len\""]
 set executable ${testfile}
 
-if {[prepare_for_testing "failed to prepare" $testfile \
-         [list $srcfile3 $srcfile $srcfile2] $flags] == -1} {
+if { [prepare_for_testing_full "failed to prepare" \
+	  [list $testfile {} \
+	       $srcfile [concat $flags {nodebug}]  \
+	       $srcfile2 {nodebug} \
+	       $srcfile3 {debug}]] == -1 } {
     return -1
 }