run-tests: Fix running with a relative path to the toolchain

This fixes github issue #181.

run-tests.sh

@@ -21,6 +21,7 @@ if [ $# -lt 1 ]; then
     exit 1
 fi
 PREFIX="$1"
+PREFIX="$(cd "$PREFIX" && pwd)"
 export PATH=$PREFIX/bin:$PATH
 
 : ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}