[gdb/testsuite] Drop -readnow in three gdb.dwarf2 test-cases

When running the testsuite in an enviroment simulating a stressed system, I
ran into timeouts in three test-cases in gdb.dwarf2:
- gdb.dwarf2/count.exp,
- gdb.dwarf2/implptrconst.exp, and
- gdb.dwarf2/implptrpiece.exp.

In all three cases, -readnow is used which results in symtabs being expanded for
the executable, /lib64/libc.so.6 and /lib64/ld-linux-x86-64.so.2.

We could address this by limiting the scope of -readnow to the executable, but
after reviewing the test-cases there doesn't seem to be a clear reason to use
-readnow.

Fix this by dropping the -readnow.

Tested on x86_64-linux.

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

gdb/testsuite/gdb.dwarf2/count.exp

@@ -124,25 +124,11 @@ Dwarf::assemble $asm_file {
     }
 }
 
-if { [gdb_compile ${srcdir}/${subdir}/${srcfile} ${binfile}1.o \
-	  object {nodebug}] != "" } {
+if { [prepare_for_testing "failed to prepare" $testfile \
+	  [list $srcfile $asm_file] {nodebug}] } {
     return -1
 }
 
-if { [gdb_compile $asm_file ${binfile}2.o object {nodebug}] != "" } {
-    return -1
-}
-
-if { [gdb_compile [list ${binfile}1.o ${binfile}2.o] \
-	  "${binfile}" executable {}] != "" } {
-    return -1
-}
-
-save_vars { GDBFLAGS } {
-    set GDBFLAGS [concat $GDBFLAGS " -readnow"]
-    clean_restart ${testfile}
-}
-
 if ![runto_main] {
     return -1
 }

gdb/testsuite/gdb.dwarf2/implptrconst.exp

@@ -83,18 +83,11 @@ Dwarf::assemble $asm_file {
     }
 }
 
-if { [build_executable ${testfile}.exp ${testfile} \
+if { [prepare_for_testing "failed to prepare" ${testfile} \
 	  [list $srcfile $asm_file] {nodebug}] } {
     return -1
 }
 
-# We need --readnow because otherwise we never read in the CU we
-# created above.
-save_vars { GDBFLAGS } {
-    set GDBFLAGS "$GDBFLAGS -readnow"
-    clean_restart ${testfile}
-}
-
 if ![runto_main] {
     return -1
 }

gdb/testsuite/gdb.dwarf2/implptrpiece.exp

@@ -101,18 +101,11 @@ Dwarf::assemble $asm_file {
     }
 }
 
-if { [build_executable ${testfile}.exp ${testfile} \
+if { [prepare_for_testing "failed to prepare" ${testfile} \
 	  [list $srcfile $asm_file] {nodebug}] } {
     return -1
 }
 
-# We need --readnow because otherwise we never read in the CU we
-# created above.
-save_vars { GDBFLAGS } {
-    set GDBFLAGS "$GDBFLAGS -readnow"
-    clean_restart ${testfile}
-}
-
 if ![runto_main] {
     return -1
 }