go/internal/gcimporter: add a test case for issue 51219
go/internal/gcimporter: add a test case for issue 51219

Based on CL 387814, construct a reproducer for issue 51219 using
x/tools/go/internal/gcimporter.

Confirmed that this fails without the fix in CL 392475.

Additionally, this CL includes some minor cleanup:
 - no need to take two passes in testExportSrc, as
   IExportVersionGenerics and IExportVersionGo1_18 are the same
 - remove skips that are no longer needed.

Fixes golang/go#51219

Change-Id: Ia76fe9038aab7a2b9efc8429dc211b03adbb5560
Reviewed-on: https://go-review.googlesource.com/c/tools/+/392734
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools