tools_common.sh: add directory name to error output

+ add a helper function to reduce the duplication

this is a bit clearer when the environment variable is set, but the
directory is missing

Change-Id: I08f9b56122b5741bb40a5f795f7f82f5b49f1047

test/tools_common.sh

diff --git a/test/tools_common.sh b/test/tools_common.sh
index 8e8d785..dffd4c4 100755
--- a/test/tools_common.sh
+++ b/test/tools_common.sh
@@ -106,22 +106,24 @@
   fi
 }
 
+# $1 is the name of an environment variable containing a directory name to
+# test.
+test_env_var_dir() {
+  local dir=$(eval echo "\${$1}")
+  if [ ! -d "${dir}" ]; then
+    elog "'${dir}': No such directory"
+    elog "The $1 environment variable must be set to a valid directory."
+    return 1
+  fi
+}
+
 # This script requires that the LIBVPX_BIN_PATH, LIBVPX_CONFIG_PATH, and
 # LIBVPX_TEST_DATA_PATH variables are in the environment: Confirm that
 # the variables are set and that they all evaluate to directory paths.
 verify_vpx_test_environment() {
-  if [ ! -d "${LIBVPX_BIN_PATH}" ]; then
-    echo "The LIBVPX_BIN_PATH environment variable must be set."
-    return 1
-  fi
-  if [ ! -d "${LIBVPX_CONFIG_PATH}" ]; then
-    echo "The LIBVPX_CONFIG_PATH environment variable must be set."
-    return 1
-  fi
-  if [ ! -d "${LIBVPX_TEST_DATA_PATH}" ]; then
-    echo "The LIBVPX_TEST_DATA_PATH environment variable must be set."
-    return 1
-  fi
+  test_env_var_dir "LIBVPX_BIN_PATH" \
+    && test_env_var_dir "LIBVPX_CONFIG_PATH" \
+    && test_env_var_dir "LIBVPX_TEST_DATA_PATH"
 }
 
 # Greps vpx_config.h in LIBVPX_CONFIG_PATH for positional parameter one, which