[TESTS] Import script robustness (set -u) (#2896)

Adopt the "set -u" idiom from the docker scripts as a mechanism to
improve future robustness.

tests/scripts/task_cpp_unittest.sh

 #!/bin/bash
 
 set -e
+set -u
 
-export LD_LIBRARY_PATH=lib:${LD_LIBRARY_PATH}
+export LD_LIBRARY_PATH="lib:${LD_LIBRARY_PATH:-}"
 
 make cpptest -j8
 for test in build/*_test; do

tests/scripts/task_golang.sh

 #!/bin/bash
 
 set -e
+set -u
 
-export LD_LIBRARY_PATH=lib:$LD_LIBRARY_PATH
+export LD_LIBRARY_PATH="lib:${LD_LIBRARY_PATH:-}"
 
 tvm_root="$(git rev-parse --show-toplevel)"
 export PYTHONPATH="$tvm_root/python":"$tvm_root/nnvm/python":"$tvm_root/topi/python"

tests/scripts/task_java_unittest.sh

 #!/bin/bash
 
 set -e
+set -u
 
 export PYTHONPATH=python
-export LD_LIBRARY_PATH=lib:${LD_LIBRARY_PATH}
+export LD_LIBRARY_PATH="lib:${LD_LIBRARY_PATH:-}"
 
 CURR_DIR=$(cd `dirname $0`; pwd)
 SCRIPT_DIR=$CURR_DIR/../../jvm/core/src/test/scripts

tests/scripts/task_python_integration.sh

 #!/bin/bash
 
 set -e
+set -u
 
 export PYTHONPATH=python:topi/python:apps/extension/python
-export LD_LIBRARY_PATH=build:${LD_LIBRARY_PATH}
+export LD_LIBRARY_PATH="build:${LD_LIBRARY_PATH:-}"
 
 rm -rf python/tvm/*.pyc python/tvm/*/*.pyc python/tvm/*/*/*.pyc
 

tests/scripts/task_rust.sh

 #!/bin/bash
 
 set -e
+set -u
 
 export TVM_HOME="$(git rev-parse --show-toplevel)"
 
-export LD_LIBRARY_PATH="$TVM_HOME/lib":"$TVM_HOME/build":"$TVM_HOME/nnvm":$LD_LIBRARY_PATH
+export LD_LIBRARY_PATH="$TVM_HOME/lib:$TVM_HOME/build:$TVM_HOME/nnvm:${LD_LIBRARY_PATH:-}"
 export PYTHONPATH="$TVM_HOME/python":"$TVM_HOME/nnvm/python":"$TVM_HOME/topi/python"
 export RUST_DIR="$TVM_HOME/rust"