tool/gocross: remove trimpath from test builds

trimpath can be inconvenient for IDEs and LSPs that do not always
correctly handle module relative paths, and can also contribute to
caching bugs taking effect. We rarely have a real need for trimpath of
test produced binaries, so avoiding it should be a net win.

Updates #2988
Signed-off-by: James Tucker <james@tailscale.com>

tool/gocross/autoflags.go

@@ -35,7 +35,7 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		cc          = "cc"
 		targetOS    = cmp.Or(env.Get("GOOS", ""), nativeGOOS)
 		targetArch  = cmp.Or(env.Get("GOARCH", ""), nativeGOARCH)
-		buildFlags  = []string{"-trimpath"}
+		buildFlags  = []string{}
 		cgoCflags   = []string{"-O3", "-std=gnu11", "-g"}
 		cgoLdflags  []string
 		ldflags     []string
@@ -47,6 +47,10 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		subcommand = argv[1]
 	}
 
+	if subcommand != "test" {
+		buildFlags = append(buildFlags, "-trimpath")
+	}
+
 	switch subcommand {
 	case "build", "env", "install", "run", "test", "list":
 	default:

tool/gocross/autoflags_test.go

@@ -163,7 +163,6 @@ GOTOOLCHAIN=local (was <nil>)
 TS_LINK_FAIL_REFLECT=0 (was <nil>)`,
 			wantArgv: []string{
 				"gocross", "test",
-				"-trimpath",
 				"-tags=tailscale_go,osusergo,netgo",
 				"-ldflags", "-X tailscale.com/version.longStamp=1.2.3-long -X tailscale.com/version.shortStamp=1.2.3 -X tailscale.com/version.gitCommitStamp=abcd -X tailscale.com/version.extraGitCommitStamp=defg '-extldflags=-static'",
 				"-race",