tests: Fix shellcheck issue SC2148 by adding missing shell directive

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
diff --git a/tests/common b/tests/common
index 92a1830..8bb62ff 100644
--- a/tests/common
+++ b/tests/common
@@ -1,4 +1,6 @@
 
+# shellcheck shell=bash
+
 SWTPM=swtpm
 SWTPM_EXE=${SWTPM_EXE:-${ROOT}/src/swtpm/${SWTPM}}
 SWTPM_IOCTL=${SWTPM_IOCTL:-${ROOT}/src/swtpm_ioctl/swtpm_ioctl}