Use TEST_SRCDIR for shell tests

Fixes https://github.com/bazelbuild/bazel-skylib/issues/143

tests/analysis_test_test.sh

diff --git a/tests/analysis_test_test.sh b/tests/analysis_test_test.sh
index 476fd41..5dc52b8 100755
--- a/tests/analysis_test_test.sh
+++ b/tests/analysis_test_test.sh
@@ -150,4 +150,5 @@
   expect_log "This rule should never work"
 }
 
+cd "$TEST_TMPDIR"
 run_suite "analysis_test test suite"

tests/unittest_test.sh

diff --git a/tests/unittest_test.sh b/tests/unittest_test.sh
index e4c8008..5b9d43f 100755
--- a/tests/unittest_test.sh
+++ b/tests/unittest_test.sh
@@ -131,4 +131,5 @@
   expect_log "Expected failure of target_under_test, but found success"
 }
 
+cd "$TEST_TMPDIR"
 run_suite "unittest test suite"