[gdb/testsuite] Fix gdb.dwarf2/dw-form-strx-out-of-bounds.exp with make-check-all.sh

I forgot to run test-case gdb.dwarf2/dw-form-strx-out-of-bounds.exp with
make-check-all.sh, and consequently failed to notice that it fails with for
instance target board fission-dwp.

The test-case does:
...
source $srcdir/$subdir/dw-form-strx.exp.tcl
...
and in that tcl file, prepare_for_testing fails, so a -1 is returned, but
that is ignored by the source command.

Fix this by using require, but rather that testing the result of the source
command, communicate success by setting a global variable
prepare_for_testing_done.

Likewise in gdb.dwarf2/dw-form-strx.exp.

Also, the test-case gdb.dwarf2/dw-form-strx-out-of-bounds.exp fails for target
board readnow, because the DWARF error occurs during a different command than
expected.

Fix this by just skipping the test-case in that case.

Tested on x86_64-linux.

Reported-by: Simon Marchi <simark@simark.ca>
Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.dwarf2/dw-form-strx-out-of-bounds.exp

@@ -18,7 +18,13 @@
 # Out of bounds index.
 set int_str_idx 1
 
+# With readnow, the dwarf error is printed during the file command, so skip
+# the test.
+require !readnow
+
+set prepare_for_testing_done 0
 source $srcdir/$subdir/dw-form-strx.exp.tcl
+require {expr $prepare_for_testing_done == 1}
 
 set re_dwarf_error \
     [string_list_to_regexp \

gdb/testsuite/gdb.dwarf2/dw-form-strx.exp

@@ -18,6 +18,8 @@
 # Correct index.
 set int_str_idx 0
 
+set prepare_for_testing_done 0
 source $srcdir/$subdir/dw-form-strx.exp.tcl
+require {expr $prepare_for_testing_done == 1}
 
 gdb_test "ptype global_var" "type = int"

gdb/testsuite/gdb.dwarf2/dw-form-strx.exp.tcl

@@ -56,5 +56,9 @@ Dwarf::assemble $asm_file {
 
 if { [prepare_for_testing "failed to prepare" ${testfile} \
 	  [list $srcfile $asm_file] {nodebug}] } {
-    return -1
+    return
 }
+
+# Let includers know prepare_for_testing was done, without having to check
+# source return status.
+set prepare_for_testing_done 1