[gdb/testsuite] Set EDITOR to true before using edit

The test-case gdb.base/list-ambiguous.exp normally passes, but with target
board readnow, some tests fail.

In particular, for this test, edit doesn't fail as expected:
...
    # While at it, test the "edit" command as well, since it shares
    # code with "list".
    gdb_test "edit $symbol" \
        "Specified line is ambiguous:\r\n${h0_re}\r\n${h1_re}"
...
and the editor is launched, in my case:
...
$ echo $EDITOR
/home/vries/bin/emacs-nw.sh
...
which result in all subsequent tests failing with timeout, and an editor
backup file in my sources:
...
$ git status --ignored
On branch master
Ignored files:
  (use "git add -f <file>..." to include in what will be committed)

        gdb/testsuite/gdb.base/#list-ambiguous0.c#

nothing to commit, working tree clean
...

Fix this by setting EDITOR to true before starting gdb in this test-case.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.base/list-ambiguous.exp: Set EDITOR to true.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-11  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.base/list-ambiguous.exp: Set EDITOR to true.
+
 2020-03-11  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.ada/access_to_packed_array.exp: Test printing of expanded

gdb/testsuite/gdb.base/list-ambiguous.exp

@@ -18,6 +18,10 @@
 
 standard_testfile list-ambiguous0.c list-ambiguous1.c
 
+# Set EDITOR to true to prevent that GDB's edit command starts an actual
+# editor.
+setenv EDITOR true
+
 if {[prepare_for_testing "failed to prepare" $testfile [list $srcfile $srcfile2] \
 	 {debug}]} {
     return -1