build: mv lib/libvarnishapi/vsl_glob_test_coverage{,.sh}

Trying to unconfuse bmake on FreeBSD:

    make[6]: don't know how to make vsl_glob_test_coverage. Stop

But I can't reproduce it locally with bmake on Linux.

lib/libvarnishapi/Makefile.am

@@ -91,6 +91,7 @@ vsl_glob_test_SOURCES = vsl_glob_test.c
 vsl_glob_test_CFLAGS = @SAN_CFLAGS@
 vsl_glob_test_LDADD = libvarnishapi.la @SAN_LDFLAGS@
 
-dist_noinst_SCRIPTS = vsl_glob_test_coverage
+dist_noinst_SCRIPTS = vsl_glob_test_coverage.sh
 
-TESTS = vsl_glob_test_coverage
+TESTS = vsl_glob_test_coverage.sh
+TEST_EXTENSIONS = .sh