cmd/go: document that tests run in the source directory

Fixes #13538.

Change-Id: I621bbe2befe838d16d3664d7a5e30d5d7cceae33
Reviewed-on: https://go-review.googlesource.com/17949Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -1486,6 +1486,11 @@ execution, not to the test itself.)
 The test flags that generate profiles (other than for coverage) also
 leave the test binary in pkg.test for use when analyzing the profiles.
 
+When 'go test' runs a test binary, it does so from within the
+corresponding package's source code directory. Depending on the test,
+it may be necessary to do the same when invoking a generated test
+binary directly.
+
 The command-line package list, if present, must appear before any
 flag not known to the go test command. Continuing the example above,
 the package list would have to appear before -myflag, but could appear

src/cmd/go/test.go

@@ -256,6 +256,11 @@ execution, not to the test itself.)
 The test flags that generate profiles (other than for coverage) also
 leave the test binary in pkg.test for use when analyzing the profiles.
 
+When 'go test' runs a test binary, it does so from within the 
+corresponding package's source code directory. Depending on the test,
+it may be necessary to do the same when invoking a generated test
+binary directly.
+
 The command-line package list, if present, must appear before any
 flag not known to the go test command. Continuing the example above,
 the package list would have to appear before -myflag, but could appear