gdb/testsuite: don't use source tree as temporary HOME directory

gdb/testsuite: don't use source tree as temporary HOME directory

In this commit:

  commit 1845e254645efbc02248345ccdb557d265dd8ae1
  Date:   Wed May 5 16:50:17 2021 +0100

      gdb/guile: perform tilde expansion when sourcing guile scripts

A test was added that tries to source a guile script from the users
HOME directory.  In order to achieve this the test (temporarily)
modifies $HOME to point into the binutils-gdb source tree.

The problem with this is that sourcing a guile script can cause the
guile script to be byte compiled and written into a .cache/ directory,
which is stored .... in the $HOME directory.

The result was that the test added in the above commit would cause a
.cache/ directory to be added into the binutils-gdb source tree.

In this commit the test is updated to create a new directory in the
build tree, the file we want to source is copied over, and $HOME is
set to point at the location in the build tree.  Now when the test is
run the .cache/ directory is created in the build tree, leaving the
source tree untouched.

gdb/testsuite/ChangeLog:

	* gdb.guile/guile.exp: Don't use the source directory as a
	temporary HOME directory.