Makefile: fix generating coverage with TESTPKGS
Makefile: fix generating coverage with TESTPKGS

This commit fixes the problem where the package that is passed to
`-coverpkg` is not full package path, meaning having the prefix of
"github.com/cilium/cilium". For example, if I wanted to test the `eni`
package, I would need to pass "github.com/cilium/cilium/pkg/aws/eni".
Before this commit, the value passed is "pkg/aws/eni".

Before:
$ make SKIP_KVSTORES=true TESTPKGS=pkg/rand unit-tests
...
for pkg in github.com/cilium/cilium/pkg/rand; do \
        go test -mod=vendor ... $pkg -test.v -timeout 360s -coverprofile=coverage.out -covermode=count -coverpkg pkg/rand || exit 1; \
        tail -n +2 coverage.out >> coverage-all-tmp.out; \
done
warning: no packages being tested depend on matches for pattern pkg/rand
=== RUN   Test
OK: 1 passed
--- PASS: Test (0.00s)
=== RUN   TestIntn
--- PASS: TestIntn (0.00s)
PASS
coverage: [no statements]
ok      github.com/cilium/cilium/pkg/rand       0.003s  coverage: [no statements]
make generate-cov
make[1]: Entering directory '/home/chris/code/cilium/cilium'
dirname: missing operand
Try 'dirname --help' for more information.
\# Remove generated code from coverage
grep -Ev '(^github.com/cilium/cilium/api/v1)|(generated.deepcopy.go)|(^github.com/cilium/cilium/pkg/k8s/client/)' \
        coverage-all-tmp.out > coverage-all.out
go tool cover -html=coverage-all.out -o=coverage-all.html
rm coverage.out coverage-all-tmp.out
make[1]: Leaving directory '/home/chris/code/cilium/cilium'
make stop-kvstores
...

After:
$ make SKIP_KVSTORES=true TESTPKGS=pkg/rand unit-tests
...
for pkg in github.com/cilium/cilium/pkg/rand; do \
        go test -mod=vendor ... $pkg -test.v -timeout 360s \
        -coverpkg $pkg -coverprofile=coverage.out -covermode=count || exit 1; \
        tail -n +2 coverage.out >> coverage-all-tmp.out; \
done
=== RUN   Test
OK: 1 passed
--- PASS: Test (0.00s)
=== RUN   TestIntn
--- PASS: TestIntn (0.00s)
PASS
coverage: 36.1% of statements in github.com/cilium/cilium/pkg/rand
ok      github.com/cilium/cilium/pkg/rand       0.006s  coverage: 36.1% of statements in github.com/cilium/cilium/pkg/rand
make generate-cov
make[1]: Entering directory '/home/chris/code/cilium/cilium'
dirname: missing operand
Try 'dirname --help' for more information.
\# Remove generated code from coverage
grep -Ev '(^github.com/cilium/cilium/api/v1)|(generated.deepcopy.go)|(^github.com/cilium/cilium/pkg/k8s/client/)' \
        coverage-all-tmp.out > coverage-all.out
go tool cover -html=coverage-all.out -o=coverage-all.html
rm coverage.out coverage-all-tmp.out
make[1]: Leaving directory '/home/chris/code/cilium/cilium'
make stop-kvstores
...

Signed-off-by: Chris Tarazi <711c73f64afdce07b7e38039a96d2224209e9a6c@isovalent.com>

cilium/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium-team/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium