cmd/promtool: resolve relative paths in alert test files (#5336)

Like `promtool check config <path/to/foo.yaml>`, which resolves relative
paths inside foo.yaml to be relative to `path/to`, this now makes
`promtool test rules <path/to/test.yaml>` do the same thing.

Signed-off-by: David Symonds <dsymonds@gmail.com>

cmd/promtool/unittest.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"reflect"
 	"sort"
 	"strconv"
@@ -68,6 +69,7 @@ func ruleUnitTest(filename string) []error {
 	if err := yaml.UnmarshalStrict(b, &unitTestInp); err != nil {
 		return []error{err}
 	}
+	resolveFilepaths(filepath.Dir(filename), &unitTestInp)
 
 	if unitTestInp.EvaluationInterval == 0 {
 		unitTestInp.EvaluationInterval = 1 * time.Minute
@@ -125,6 +127,16 @@ func (utf *unitTestFile) maxEvalTime() time.Duration {
 	return maxd
 }
 
+// resolveFilepaths joins all relative paths in a configuration
+// with a given base directory.
+func resolveFilepaths(baseDir string, utf *unitTestFile) {
+	for i, rf := range utf.RuleFiles {
+		if rf != "" && !filepath.IsAbs(rf) {
+			utf.RuleFiles[i] = filepath.Join(baseDir, rf)
+		}
+	}
+}
+
 // testGroup is a group of input series and tests associated with it.
 type testGroup struct {
 	Interval        time.Duration    `yaml:"interval"`