Fix failed windows tests that depends on cwd

[youyuanwu]

Signed-off-by: Youyuan Wu youyuanwu@outlook.com

Failed tests on master:

--- FAIL: TestNormalizer_NormalizeBase (0.00s)
    --- FAIL: TestNormalizer_NormalizeBase//base/sub/file.json (0.00s)
        normalizer_test.go:[41](https://github.com/go-openapi/spec/runs/7816924392?check_suite_focus=true#step:4:42)8: 
            	Error Trace:	normalizer_test.go:418
            	Error:      	Not equal: 
            	            	expected: "file:///c:/base/sub/file.json"
            	            	actual  : "file:///d:/base/sub/file.json"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-file:///c:/base/sub/file.json
            	            	+file:///d:/base/sub/file.json
            	Test:       	TestNormalizer_NormalizeBase//base/sub/file.json
            	Messages:   	for base "/base/sub/file.json"
    --- FAIL: TestNormalizer_NormalizeBase//.. (0.00s)
        normalizer_test.go:418: 
            	Error Trace:	normalizer_test.go:418
            	Error:      	Not equal: 
            	            	expected: "file:///c:"
            	            	actual  : "file:///d:"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-file:///c:
            	            	+file:///d:
            	Test:       	TestNormalizer_NormalizeBase//..
            	Messages:   	for base "/.."
    --- FAIL: TestNormalizer_NormalizeBase////folder//subfolder///file.json/ (0.00s)
        normalizer_test.go:418: 
            	Error Trace:	normalizer_test.go:418
            	Error:      	Not equal: 
            	            	expected: "file:///c:/folder/subfolder/file.json"
            	            	actual  : "file:///d:/folder/subfolder/file.json"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-file:///c:/folder/subfolder/file.json
            	            	+file:///d:/folder/subfolder/file.json
            	Test:       	TestNormalizer_NormalizeBase////folder//subfolder///file.json/
            	Messages:   	for base "///folder//subfolder///file.json/"

The cause of failure is that the tests were running in D:/ drive in stead of C:/ drive in github action.
Not sure the filepath.abs() changed behaviour or not. i.e. if it always returns C:/.

The fix is to change tests to have expected value matching the current drive.