Skip most tests for non-author installations

Since we offer p6doc, this distro is also installed by users
who want that tool. There's no reason to abort those installs
just because someone used tabs in the text.

Skips all the author-related tests, including tests for helper
utilities like Pod::Convenience.

t/00-load.t

@@ -2,6 +2,7 @@
 
 use v6;
 use Test;
+BEGIN plan :skip-all<Test applicable to git checkout only> unless '.git'.IO.e;
 
 plan 1;
 

t/meta.t

@@ -1,11 +1,11 @@
 #!/usr/bin/env perl6
 
 use v6;
-
 use lib 'lib';
 use Test;
-use Test::META;
+BEGIN plan :skip-all<Test applicable to git checkout only> unless '.git'.IO.e;
 
+use Test::META;
 meta-ok;
 
 done-testing;

t/pod-convenience.t

@@ -1,10 +1,11 @@
 #!/usr/bin/env perl6
 
 use v6;
-use Test;
 use lib 'lib';
-use Pod::Convenience;
+use Test;
+BEGIN plan :skip-all<Test applicable to git checkout only> unless '.git'.IO.e;
 
+use Pod::Convenience;
 plan 10;
 
 subtest {

t/pod-htmlify.t

@@ -3,6 +3,7 @@
 use v6;
 use Test;
 use lib 'lib';
+BEGIN plan :skip-all<Test applicable to git checkout only> unless '.git'.IO.e;
 use File::Temp;
 
 BEGIN {

t/pod-valid.t

@@ -1,8 +1,10 @@
 #!/usr/bin/env perl6
 
 use v6;
-use Test;
 use lib 'lib';
+use Test;
+BEGIN plan :skip-all<Test applicable to git checkout only> unless '.git'.IO.e;
+
 use Test-Files;
 
 =begin overview

t/pod6.t

@@ -1,8 +1,10 @@
 #!/usr/bin/env perl6
 
 use v6;
-use Test;
 use lib 'lib';
+use Test;
+BEGIN plan :skip-all<Test applicable to git checkout only> unless '.git'.IO.e;
+
 use Test-Files;
 
 my @files = Test-Files.files;

t/tabs.t

@@ -1,8 +1,9 @@
 #!/usr/bin/env perl6
 
 use v6;
-use Test;
 use lib 'lib';
+use Test;
+BEGIN plan :skip-all<Test applicable to git checkout only> unless '.git'.IO.e;
 use Test-Files;
 
 my @files = Test-Files.files\