cmd/go: respect gcflags, ldflags in 'go test'

Fixes bug introduced by https://golang.org/cl/129059 where
gcflags='all=...' and ldflags='all=...' would not be applied to some
packages built by 'go test'.

LoadImport used to set gcflags/ldflags for the Package objects it
created, in https://golang.org/cl/129059 this code was factored out to
setToolFlags. The codepath of `go build` was updated to call
setToolFlags appropriatley, but the codepath of `go test -c` wasn't,
resulting in gcflags/ldflags being applied inconsistently when building
tests.

This commit changes TestPackagesFor to call setToolFlags on the package
objects it creates.

Fixes #27681

Change-Id: Idcbec0c989ee96ec066207184611f08818873e8d
Reviewed-on: https://go-review.googlesource.com/c/136275
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/load/test.go

@@ -227,6 +227,12 @@ func TestPackagesFor(p *Package, cover *TestCover) (pmain, ptest, pxtest *Packag
 		}
 	}
 
+	allTestImports := make([]*Package, 0, len(pmain.Internal.Imports)+len(imports)+len(ximports))
+	allTestImports = append(allTestImports, pmain.Internal.Imports...)
+	allTestImports = append(allTestImports, imports...)
+	allTestImports = append(allTestImports, ximports...)
+	setToolFlags(allTestImports...)
+
 	// Do initial scan for metadata needed for writing _testmain.go
 	// Use that metadata to update the list of imports for package main.
 	// The list of imports is used by recompileForTest and by the loop

src/cmd/go/testdata/script/gcflags_patterns.txt

@@ -21,6 +21,10 @@ stderr 'compile.* -p y'
 go build -n -v -gcflags='  z1 =  	-e 	' z1
 stderr 'compile.* -e .*-p z1'
 
+# -gcflags='all=-N -l' should apply to all packages, even with go test
+go test -c -n -gcflags='all=-N -l' z1
+stderr 'compile.* -N -l .*-p z3 '
+
 # -ldflags for implicit test package applies to test binary
 go test -c -n -gcflags=-N -ldflags=-X=x.y=z z1
 stderr 'compile.* -N .*z_test.go'
@@ -58,11 +62,15 @@ import _ "z2"
 -- z1/z_test.go --
 package z1_test
 import "testing"
+import _ "z3"
 func Test(t *testing.T) {}
 
 -- z2/z.go --
 package z2
 
+-- z3/z.go --
+package z3
+
 -- y/y.go --
 package y