[gdb/testsuite] Fix gdb.base/dlmopen.exp on native-gdbserver

With test-case gdb.base/dlmopen.exp and target board native-gdbserver, I get:
...
(gdb) info breakpoints 3^M
Num     Type           Disp Enb Address            What^M
3       breakpoint     keep y   0x00007ffff7fc8000 ^M
        stop only if (0) (target evals)^M
(gdb) FAIL: $exp: test_solib_unmap_events: check b/p status
...

The problem is that the regexp doesn't allow for the "(target evals)" part:
...
	-re -wrap "$bpnum\\s+breakpoint\\s+keep\\s+y\\s+$::hex\\s*\[^\r\n\]+\r\n\\s+stop o
nly if \\(0\\)" {
...

Fix this by updating the regexp.

While we're at it:
- rewrite the regexp using multiline, string_to_regexp, join and string cat to
  make it more readable, and
- remove the redundant failure clause from the same gdb_test_multiple.

Tested on x86_64-linux using make-check-all.sh.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.base/dlmopen.exp

@@ -358,15 +358,19 @@ proc_with_prefix test_solib_unmap_events { } {
     # dynamic linker as pending when some instances of the library were
     # unloaded, despite there really only being one copy of the dynamic
     # linker actually loaded into the inferior's address space.
-    gdb_test_multiple "info breakpoints $bpnum" "check b/p status" {
-	-re -wrap "$bpnum\\s+breakpoint\\s+keep\\s+y\\s+<PENDING>\\s+\\*$::hex\\s*\r\n\\s+stop only if \\(0\\)" {
-	    fail $gdb_test_name
-	}
-
-	-re -wrap "$bpnum\\s+breakpoint\\s+keep\\s+y\\s+$::hex\\s*\[^\r\n\]+\r\n\\s+stop only if \\(0\\)" {
-	    pass $gdb_test_name
-	}
-    }
+    set hs {[^\r\n]}
+    set re_pass \
+	[multi_line \
+	     "" \
+	     [join \
+		  [list \
+		       "$bpnum" "breakpoint" "keep" "y" "$::hex$hs+"] \
+		  {\s+}] \
+	     [string cat \
+		  {\s+} \
+		  [string_to_regexp "stop only if (0)"] \
+		  ([string_to_regexp " (target evals)"])?]]
+    gdb_test "info breakpoints $bpnum" $re_pass "check b/p status"
 
     # With all the dlclose calls now complete, we should be back to a
     # single copy of the dynamic linker.