[gdb/testsuite] Fix imported-unit.exp FAIL without psymtabs

The test-case gdb.dwarf2/imported-unit.exp contains a test testing partial
symbols, so when we run the test-case using either target board readnow,
cc-with-gdb-index or cc-with-debug-names, we run into:
...
FAIL: gdb.dwarf2/imported-unit.exp: no static partial symbols in importing unit
...

Fix this by marking the test unsupported if there are no partial symbols.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-08  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (psymtabs_p): New proc.
	* gdb.dwarf2/imported-unit.exp: Mark "no static partial symbols in
	importing unit" unsupported if there are no partial symbols.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-08  Tom de Vries  <tdevries@suse.de>
+
+	* lib/gdb.exp (psymtabs_p): New proc.
+	* gdb.dwarf2/imported-unit.exp: Mark "no static partial symbols in
+	importing unit" unsupported if there are no partial symbols.
+
 2020-04-08  Tom de Vries  <tdevries@suse.de>
 
 	PR testsuite/25760

gdb/testsuite/gdb.dwarf2/imported-unit.exp

@@ -149,19 +149,25 @@ if { [prepare_for_testing "failed to prepare" ${testfile} \
 
 gdb_test_no_output "set language c++"
 
+set psymtabs_p [psymtabs_p]
+
 # Verify that the partial symtab for the unit importing the partial unit does
 # not contain the static partial symbol int, which is defined in the partial
 # unit.  Test-case for PR25646.
-gdb_test "main print psymbols" \
-    [multi_line \
-	 "  Depends on 1 other partial symtabs\." \
-	 "\[^\r\n\]*" \
-	 "  Global partial symbols:" \
-	 "    `main', function, $hex" \
-	 "" \
-	 ".*"] \
-    "no static partial symbols in importing unit"
-
+set test "no static partial symbols in importing unit"
+if { $psymtabs_p } {
+    gdb_test "main print psymbols" \
+	[multi_line \
+	     "  Depends on 1 other partial symtabs\." \
+	     "\[^\r\n\]*" \
+	     "  Global partial symbols:" \
+	     "    `main', function, $hex" \
+	     "" \
+	     ".*"] \
+	$test
+} else {
+    unsupported $test
+}
 # Sanity check
 gdb_test "ptype main" "= int \\(void\\)"
 

gdb/testsuite/lib/gdb.exp

@@ -6998,6 +6998,24 @@ proc readnow { } {
     return 0
 }
 
+# Return 1 if partial symbols are available.  Otherwise, return 0.
+
+proc psymtabs_p {  } {
+    global gdb_prompt
+
+    set cmd "maint info psymtab"
+    gdb_test_multiple $cmd "" {
+	-re "$cmd\r\n$gdb_prompt $" {
+	    return 0
+	}
+	-re -wrap "" {
+	    return 1
+	}
+    }
+
+    return 0
+}
+
 # Verify that partial symtab expansion for $filename has state $readin.
 
 proc verify_psymtab_expanded { filename readin } {