testsuite, trace: add guards if In-Process Agent library is not found

Several tests in gdb.trace trigger TCL errors if the In-Process Agent
library is not found, e.g.:

  Running gdb/testsuite/gdb.trace/change-loc.exp ...
  ERROR: tcl error sourcing gdb/testsuite/gdb.trace/change-loc.exp.
  ERROR: error copying "gdb/gdb/testsuite/../../gdbserver/libinproctrace.so":
	 no such file or directory
      while executing
  "file copy -force $fromfile $tofile"
      (procedure "gdb_remote_download" line 29)
      invoked from within
  "gdb_remote_download target $target_file"
      (procedure "gdb_download_shlib" line 6)
      invoked from within
  "gdb_download_shlib $file"
      (procedure "gdb_load_shlib" line 2)
      invoked from within
  "gdb_load_shlib $libipa"
      (file "gdb/testsuite/gdb.trace/change-loc.exp" line 354)
      invoked from within
  "source gdb/testsuite/gdb.trace/change-loc.exp"
      ("uplevel" body line 1)
      invoked from within
  "uplevel #0 source gdb/testsuite/gdb.trace/change-loc.exp"
      invoked from within
  "catch "uplevel #0 source $test_file_name""

Protect against this error by checking if the library is available.