reset TZ when running solver tests as some output has local time

Fixes commits 37fbafc and 1c47f37.

Author: fabled

test/basic8.test

@@ -2,5 +2,5 @@
 --no-network
 add -t .virtual
 @EXPECT
-(1/1) Installing .virtual (20190603.161426)
+(1/1) Installing .virtual (20190603.131426)
 OK: 0 MiB in 0 packages

test/basic9.test

@@ -4,5 +4,5 @@
 --test-world a
 add -t .virtual a
 @EXPECT
-(1/1) Installing .virtual (20190603.161426)
+(1/1) Installing .virtual (20190603.131426)
 OK: 0 MiB in 2 packages

test/solver.sh

@@ -4,6 +4,7 @@ get_block() {
 	awk '/^@'$1'/{p=1;next} /^@/{p=0} p{print}'
 }
 
+export TZ=UTC
 APK_TEST="../src/apk-test"
 TEST_TO_RUN="$@"