[gdb/testsuite] Fix untested messages in gdb.mi/*.exp

The effect of:
...
untested "y.exp"
...
in a gdb.x/y.exp is:
...
UNTESTED: gdb.x/y.exp: y.exp
...
which is a bit pointless.

Replace these untested messages in gdb.mi/*.exp with the usual "failed to
compile".

Likewise for an:
...
untested $testname
...
where the variable is undefined.

Tested on x86_64-linux.

gdb/testsuite/gdb.mi/gdb2549.exp

@@ -28,7 +28,7 @@ set MIFLAGS "-i=mi"
 standard_testfile basics.c
 
 if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
-     untested mi-regs.exp
+     untested "failed to compile"
      return -1
 }
 

gdb/testsuite/gdb.mi/mi-async-run.exp

@@ -18,7 +18,7 @@ load_lib mi-support.exp
 standard_testfile
 
 if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
-     untested "mi-async-run.exp"
+     untested "failed to compile"
      return -1
 }
 

gdb/testsuite/gdb.mi/mi-frame-regs.exp

@@ -26,7 +26,7 @@ standard_testfile basics.c
 
 if {[gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" \
 		 executable {debug}] != "" } then {
-     untested mi-frame-regs.exp
+     untested "failed to compile"
      return -1
 }
 

gdb/testsuite/gdb.mi/mi-fullname-deleted.exp

@@ -57,7 +57,7 @@ puts $f "int main (void) { return 0; }"
 close $f
 
 if { [gdb_compile "$srcfileabs" "${binfile}" executable {debug}] != "" } {
-    untested $testname
+    untested "failed to compile"
     return -1
 }
 

gdb/testsuite/gdb.mi/mi-vla-c99.exp

@@ -30,7 +30,7 @@ standard_testfile vla.c
 
 if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" \
                   "${binfile}" executable {debug}] != "" } {
-     untested mi-vla-basics.exp
+     untested "failed to compile"
      return -1
 }