Add missing -pthread for test support libraries

This is good in general (see how the other libraries also have long had
it, since 49fe9592a4) but in particular
needed to fix the NetBSD build.

(cherry picked from commit b23273f6a2)

tests/unit/libexpr-support/local.mk

@@ -20,4 +20,4 @@ libexpr-test-support_LIBS = \
     libstore-test-support libutil-test-support \
     libexpr libstore libutil
 
-libexpr-test-support_LDFLAGS := -lrapidcheck
+libexpr-test-support_LDFLAGS := -pthread -lrapidcheck

tests/unit/libstore-support/local.mk

@@ -18,4 +18,4 @@ libstore-test-support_LIBS = \
     libutil-test-support \
     libstore libutil
 
-libstore-test-support_LDFLAGS := -lrapidcheck
+libstore-test-support_LDFLAGS := -pthread -lrapidcheck

tests/unit/libutil-support/local.mk

@@ -16,4 +16,4 @@ libutil-test-support_CXXFLAGS += $(libutil-tests_EXTRA_INCLUDES)
 
 libutil-test-support_LIBS = libutil
 
-libutil-test-support_LDFLAGS := -lrapidcheck
+libutil-test-support_LDFLAGS := -pthread -lrapidcheck