Add test_variable_data framework that reuses the snippets

Most of these snippets are likely to have some env var that causes the
data to hold constant, while the data will otherwise be likely to vary
over time.

This framework offers a way to test those snippets, by dropping the
variable and an example value in the test/snippets/FOO.variable
one-line file.

Note that the test/snippets/syscall.c snippet is *not* expected to
vary over time (or to differ when any given variable is set), so we
simply don't add any test/snippets/syscall.variable file to avoid it
being tested in this way.

test/test_variable_data.sh

@@ -0,0 +1,54 @@
+#!/bin/bash
+
+set -e
+
+FTPL="${FAKETIME_TESTLIB:-../src/libfaketime.so.1}"
+DELAY="${DELAY:-2}"
+
+declare -A firstunset first second delayed delayedunset
+
+err=0
+for func in "$@"; do
+    if ! [ -x "./run_$func" ]; then
+        printf >&2 '%s does not exist, failing\n' "./run_$func"
+        exit 1
+    fi
+    read varname value < "snippets/$func.variable"
+    unset $varname
+    firstunset[$func]="$(env LD_PRELOAD="$FTPL" "./run_$func")"
+    first[$func]="$(env LD_PRELOAD="$FTPL" "$varname=$value" "./run_$func")"
+    second[$func]="$(env LD_PRELOAD="$FTPL" "$varname=$value" "./run_$func")"
+    if [ "${first[$func]}" != "${second[$func]}" ]; then
+        printf >&2 '[%s] Set %s="%s", but got two different outputs:\n - %s\n - %s\n' "$func" "$varname" "$value" "${first[$func]}" "${second[$func]}"
+        err=$(( $err + 1 ))
+    fi
+    if [ "${first[$func]}" == "${firstunset[$func]}" ]; then
+        printf >&2 '[%s] Same answer when %s="%s" and when unset:\n -   set: %s\n - unset: %s\n' "$func" "$varname" "$value" "${first[$func]}" "${firstunset[$func]}"
+        err=$(( $err + 1 ))
+    fi
+done
+
+printf "Sleeping %d seconds..." "$DELAY"
+sleep "$DELAY"
+printf 'done\n'
+
+for func in "$@"; do
+    read varname value < "snippets/$func.variable"
+    unset $varname
+    delayed[$func]="$(env LD_PRELOAD="$FTPL" "$varname=$value" "./run_$func")"
+    delayedunset[$func]="$(LD_PRELOAD="$FTPL" "./run_$func")"
+    if [ "${first[$func]}" != "${delayed[$func]}" ]; then
+        printf >&2 '[%s] Vary across delay of %d seconds (%s="%s"):\n - before: %s\n -  after: %s\n' "$func" "$DELAY" "$varname" "$value" "${first[$func]}" "${delayed[$func]}"
+        err=$(( $err + 1 ))
+    fi
+    if [ "${firstunset[$func]}" == "${delayedunset[$func]}" ]; then
+        printf >&2 '[%s] Same answer when unset across delay of %d seconds:\n - before: %s\n -  after: %s\n' "$func" "$DELAY" "${firstunset[$func]}" "${delayedunset[$func]}"
+        err=$(( $err + 1 ))
+    fi
+done
+
+if [ "$err" -gt 0 ]; then
+    printf >&2 'Got %d errors, failing\n' "$err"
+    exit 1
+fi
+exit 0